Cross-posted to WikiAnswers. I’m archiving it here in case it gets vandalized.
The brake switch itself looks like a drumstick (in terms of chicken). The way it works is thus– there is a small white button on the long end of it. The brake lights turn off when the button is pressed, and the brake lights come on when the button is not pressed.
This may seem a little counter-intuitive, but when the switch is correctly installed, it is installed in such a way that the button is always depressed (thus keeping the brake lights off) until you step on the brake pedal. Once you step on the brake pedal, contact between the pedal and the button is released, which is what allows your brake lights to come on when you’re braking. This is important because if your brake lights are stuck in the “on” position after you replace the switch, understanding how it’s supposed to work will help you understand what you did wrong.
Now, for the actual procedure:
Recline your driver seat all the way flat. You will be laying flat on your back, on the chair, with your head down by where the pedals are and your feet up by the headrest of your seat. You may want to remove your shoes and vacuum the floor area before proceeding.
Look at the rubber part of the brake pedal (where your foot normally comes into contact with it). Now follow the arm connected to it upward. You will see a gold hinge upon which the brake pedal connects to the rest of the system; continue past it. Just past that is a black cylinder mounted in a silver bracket; the cylinder is probably angled perpendicular to the brake pedal arm. Check the Related Links section for pictures.
The brake light switch is inside this cylinder. To remove the switch, you need to look on the side of the metal plate that has wires sticking out of it. Grab the squared black part and simply twist it. Do not force it; if it isn’t turning, try it the other way. Then disconnect the white wiring harness from it, and you will have successfully extracted the brake switch.
Reinstallation is the tricky part. If you don’t do it right, your brake lights will be stuck in the “on” position.
Go ahead and insert the new switch into the black cylindrical holster. Make sure you push it in as deep as it will go, then twist it opposite in the direction you turned it to remove it. If you have installed it correctly, you should see no part of the white button on the switch unless you’re pushing on the brake pedal. The white button should be completely depressed by the little mushroom cap on the brake pedal arm.
If you can see the white button when the brake pedal is not depressed then you did not insert the switch far enough. Check to see if your brake lights are on. If they are, remove the switch and try again. If they aren’t, you might be able to get away with it but I’d recommend trying again as well.
Hook the wiring harness back up to the switch and you’re done.