Start by figuring out what the cause of the fault it, then go from there. Check for power at the lights. If you don't have that, check for continuity in the switch. If you don't have that, replace the switch.
check fuses & brake lights switch located on brake pedal
check by the pedals to the left if should be in the paneling there will be a little sticker pointing it out. you'll have to push it up if it has been engaged
not charging the battery properly... check the alternator.
check your fuses check ignition switch
What is the problem you are having with the switch?
If your check engine light is flashing , an engine cylinder misfire has been detected
check your ignition switch.
Check the ignition fuse also check the ignition switch
If you're referring to the Ford Contour/Mercury Mystique, check sites such as eBay, Craigslist, local salvage yards, etc.
Take a continuity tester and check them. you'll find the bad one
Check the Auto Zone website it gives you detail infor How to do it