I havew a 1989 mercury grand marquis. When the park brake is put on the brake indicator light does not come on. Changed the bulb but no luck. What else coul it be.

Corrosion, bad/dirty switch, or blown fuse. Here's how to test all under-dash and under-hood fuses with a test light or multimeter:

https://www.askmehelpdesk.com/cars-t...tml#post252145

It should only take several minutes to test the fuses. Use your multimeter or test light to check out the parking brake switch. Spray the switch afterwards with WD-40.