1993 Buick Regal driver side brake lights

Does the turn signal work on the back driver side? On some systems, the turn signal and brake lite use the same bulb filament, just not at the same time.

You might use a testlite on the bulb socket, check voltage, ground, if they're ok, it might be the bulb isn't making good contact in the socket. You might try a dab of lub-grease on the very bottom tip of the bulb, see if that helps.

Maybe you need a new socket.