1989 mercury grand marquis battery is fine and so is alternator why won't it start?

First off, does it turn over (crank) and not start? If it does and the motor sounds like it should start and it doesn't then check and see if your car is equipped with an ignition module, which I'm pretty sure it does. If it has an ignition module, replace that and it should solve your problem. Ignition modules are inexpensive and easy to install.