1988 crown Victoria Tail and parking lights don't work but fuse and switch is okay what could be wrong?

Tail lites work, do flashers work ? Is the bulb good ? Try a new bulb first, still does not work. Check the wire just before the lites for power with a 12 volt tester, if power check contacts in the bulb holder. Still does not work and no power to the lites, you will have to find the right wire up in the front of the car and check there, could be a break in the line.