I found a simple way to add a brake light switch to a bicycle brake lever. Specifically, this is for a mountain bike lever that pulls a cable for the brakes although it's possible it could work on other levers. I would not try this on super lightweight brake levers as the metal might not be thick enough to tap threads into and it might weaken the lever excessively. The switches I used can be found at radio shack and are mini momentary pushbutton switches that are normally closed (N.C.). The tools you will need are: allen and/or box wrenches/sockets, duct tape, half round metal file, pencil, tape measure, center punch, drill and bits, 7mm fine tap and tap holder, wire, solder, soldering pencil, heat shrink tubing, heat gun and electrical tape.
Step 1: Prepping the brake lever
The first thing you will need to do is remove the reach adjustment screw from the body as shown in the first three pictures. This can be done using a small allen wrench. Next you need to remove the lever (silver part) from the body (black part) if possible. This is usually accomplished by removing a pivot screw and nut or a pivot pin and clip. Pay careful attention to how it goes back together and set the lever and small parts aside. Next you want to wrap the body as shown in the picture with a layer of duct tape to protect the finish when you clamp it in the vise.