the tail lights are dual filaments. one is for the tail lights other for the brake/turn signal. could be that the brake/turn signal filaments is burned out but the tail light filaments is still good. i would look at the bulbs.
I would check the bulbs first. Like was just said, they could both be burned out on the brighter filiment. Then check to see if power is making it to the bulb sockets. I am inclined to agree with 2+2 thought that the TS switch may be the problem.
You need to test the connections at the TS plug. Replacing it is a bit of work:
Neither. The dash is welded in, and the instruments are not in the way.
To remove the TS, you need the remove the steering wheel, disconnect the wire plug under the dash, remove the wires from the pin block with a pin tool or small tube, detach the switch, and remove it.