What could be the cause of the tail lights of a 1995 Ford Taurus to not work in normal operation but work when the brakes are pressed?

There are 2 filiments in those bulbs. One for tails/turn and the other for brake lights. Try replacing one bulb first. I think there are two offest tits on the shaft of those bulbs. Be sure you insert it correctly into the socket. For that matter check to see if they are in correctly now.