Replacing the Stop Light SwitchIf you own a model year 2001 Prius or you bought the model year 2002 without cruise control, you may want to add it later. I think that the decision to have cruise control is exactly the same for the Prius as for any other car. Fortunately, adding cruise control is not very difficult as the car contains all the wiring already. Although non-Toyota kits are available, for example from Coastal Electronic Technologies, I chose to use the Toyota kit.
It's a bit of a mystery why Toyota did not offer cruise control on the Prius in model year 2001. The car has all the wiring for it and lacks only two switches. The stop light switch is single-pole and must be replaced with a two-pole switch so that the extra pole will cancel the cruise control setting. The cruise control switch itself must be mounted on the steering wheel. As if fitting the car with a single-pole switch instead of a two-pole switch wasn't strange enough, Toyota did not provide mounting points for the cruise control switch in the standard steering wheel. So, the cost of retrofitting cruise control was at first something like $600 because you needed to buy the new stoplight switch, the cruise control switch and a new steering wheel.
Resourceful people soon figured out how to modify the existing steering wheel to take the switch. The procedure devised by Ozone House is incredibly well described with step-by-step instructions and photographs.
In June of 2001, however, the cruise control kit, including the new steering wheel, became available more cheaply (see toyota-prius #11123). I suspect that Toyota regretted making things so difficult and dropped the price to that of just the switches. The list price should be around $220, but you don't have to pay even this if you shop around and use the Internet.
On Saturday, July 21, 2001, I successfully installed cruise control in my 2001 Prius. If you're moderately handy and don't want the hassle or expense of getting a dealer to do it, you should consider doing it yourself. (Of course, your mileage may vary and I can't take responsibility for what happens if you try to do it and something goes wrong.) The whole installation took me two hours. I was really, really careful at each step, so I can imagine someone who has done it before getting it done in an hour. I half followed the instructions that came with the kit but made some deviations inspired by the excellent Ozone House descriptions and some general engineering judgement.
The kit from ToyotaPartsNow, including a complete new steering wheel with cruise control switch already mounted, cost me $175 including shipping. I ordered it on Tuesday and it arrived on Thursday.
Not having an automotive tool set, I spent an additional $38 at Sears to get a 14 mm crows foot wrench, a Torx T30 driver and a steering wheel puller. I didn't bother with a torque wrench but instead tightened things up with about the same force it took me to undo them. Note that I've since discovered that $20 was far too much to spend on a steering wheel puller and many people have got the steering wheel off without one using a technique known as "bonking from behind".
Replacing the Stop Light SwitchWhat could be easier that replacing one little switch? Almost anything, it turns out. The switch points down towards the brake pedal through a mounting bracket. It screws into a nut that is welded to the bracket and is secured by a locknut that is run up against the bracket from the back. Easy enough, except that the bracket is bent up in such a way that you can't get at the locknut from the side where there is room to move and have to approach it from a very constricted space on the other side. For this reason the locknut is known in the Yahoo! discussion group toyota-prius as the "enemy nut". By searching on this text, you will be able to find many ways of dealing with it and a few mistakes.
In the photograph at right, the new double-pole switch is installed (center). You can see the bracket and how it bends up to hide the enemy nut from this, the most accessible, side. You can also see the welded nut, below the bracket, and if you look closely you can see the actual welds. The black plastic plunger sticks out of the lower end of the switch and is pressed by a tab on the brake pedal mechanism.
I decided to deal with the stop light switch first, even though it is second in the instructions. You can stop after this step and do the
steering wheel another day. Also, I can see no need to disconnect any batteries and so I didn't. Contrary to the instructions, I didn't
disassemble the dash. I simply reached up, around and over the mounting bracket with a crows foot wrench and loosened the
locknut. 
Here's a picture of my 14 mm crows foot wrench assembled on my 1/2 inch socket driver with a short extension and 3/8
inch adapter, plus the old switch:
After getting the wrench on the nut and holding it in place with one hand, I thought carefully about which way to turn (clockwise) and gave a good tug on the driver with the other. I felt the nut turn and the switch then unscrewed by hand. Maybe I was lucky. There are discussion group posts that make getting at that enemy nut look much more difficult than I found it. If you have trouble, I think a proper 3/8 inch drive would work even better than my clunky 1/2 inch drive and adapter. Some people have had good results with a stubby open-ended wrench or a normal wrench simply sawn off to a short length. Other people recommend taking apart the car somewhat to get better access. I don't know why I found it so easy.
Do not grab the threaded end of the switch with some gripping tool and try to force the switch to turn. Someone tried this and regretted it. Also, consensus is now against using extreme measures like grinding off the welded nut as recommended by Ozone House. The problem with this is that if the replacement nuts work loose the switch can move away from the pedal and result in the stop lights not coming on. While the switch is threaded into the welded nut, this can't happen as the wiring will stop the switch from turning and unscrewing very far.
After screwing in the new switch, I ran the nut up against the back of the bracket by hand and reversed the above process. Of, course, I adjusted the switch first. I decided to have the brake light come on with a pedal depression near the minimum to be sure that the cruise control would cancel with the first touch of the brake. If you don't have a helper, you can adjust the switch yourself. Throw a white cloth or towel over the top of the rear window and you'll be able to see when the high-level brake light comes on from your position in the footwell.
I did this part of the installation second. I disconnected the auxiliary battery negative side by unscrewing the chassis connection. This seemed easier and just as effective as messing with the battery terminal itself and involved less disassembly. To make sure it didn't touch the chassis and reconnect, I stuffed the end of the wire into a small plastic bag and taped it up securely. Remember to record your radio presets before disconnecting the battery because the car will forget them. I didn't disconnect the main (high voltage) battery because I knew I wouldn't be going anywhere near the high voltage wires. I waited much, much longer than the recommended 90 seconds before removing the air bag.
The rest went according to instructions. If you find the included instructions a bit terse (e.g. "remove the steering wheel using the steering wheel puller"), take a look at Ozone House's description and all will be clear. Just skip over all the stuff about modifying the old steering wheel because you have a complete new one with the switch already on it. Also, remember that "bonking from behind" or just tugging firmly appear to be viable alternatives to the puller, although I haven't tried either myself.
All electrical connectors have some locking device to make sure they don't fall apart. You need to find this and poke or squeeze it to separate plug and socket. Don't be tempted to just pull hard or you'll be in trouble.
Just to prove the whole thing wasn't a fluke, I joined Sam in helping Michelle install a Toyota cruise control in her car. We used the exact same procedure as above and finished in 90 minutes with a great deal of talking and stopping for photographs. I was particularly interested in whether I could repeat my easy conquest of the enemy nut. I did. It gave on the first try and I had the switch loose before it really dawned on the others I'd started. Tightening it up after installing the new switch took a while, until I realized I wasn't paying proper attention to getting the crows foot wrench firmly around the nut. When I concentrated on this rather than wiggling the whole assembly about, I got the nut tight no time. So, my advice is that if you can put together similar tools to mine, try just loosening the nut before you get into taking the car apart. You have to work by feel, but if you get it right it takes only seconds.
Here are posts to the Yahoo! toyota-prius group that describe other people's experiences installing cruise control kits. Most people chose the Toyota kit, but others prefer the Coastal Electronic Technologies kit, which does not require the steering wheel or air bag to be removed.
13299 - 35 minutes for second of two installations!
13803 - used a steering wheel puller
13901 - no disassembly, crow's foot wrench, steering wheel "bonked" off
15798 - disassembly, sawn off wrench, steering wheel removed by pulling firmly
16177 - 65 minutes
16645 - disassembly, steering wheel removed by tugging
16711 - used a crow's foot wrench and steering wheel puller
16910 - sawn off wrench, steering wheel "bonked" off
18437 - disassembly, steering wheel "bonked" off
18804 - by the book
19898 - disassembly, steering wheel "bonked" off
10845 -
11129 -
12166 -
19679 -