How do you test a brake light switch?

You will need a small electrical multi-tester to test the switch. Set the tester to "ohms". Disconnect the wiring from the switch. Touch one of the tester leads to one side of the switch and the other lead to the other side. Your reading should be zero. Have a friend depress the brake pedal, and the needle on the meter should swing left to right. If the needle does not swing then the switch is bad. If it swings then the problem is elsewhere in the wiring. The multi-tester can be found at hardware stores, auto parts stores and department stores.