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.
If your check engine light is flashing , an engine cylinder misfire has been detected
check your fuses check ignition switch
What is the problem you are having with the switch?
If you're referring to the Ford Contour/Mercury Mystique, check sites such as eBay, Craigslist, local salvage yards, etc.
May also want to check the module ontop of the transmission, it's located next to the battery, I'm not sure if the part's correct name is a neutral safety switch.
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
The ignition switch on a 1996 Mercury Tracer might lock up and release only when the shifter is removed from park and then put in the park position again because the sensor is getting worn. This sensor lets the ignition know that the car is in park and it is safe to remove the key. Check the shift position sensor.