You might do well to take the taillight out of the car and clean up the contacts on the taillight circuitry. That often helps quite a bit. Also clean all the contacts both 1) on the wiring harness connection, and 2) on the taillight, where the wiring harness plugs in.

Both of them will cause a multitude of bizarre operation.

Also, a bad ground will cause trouble galore with the taillights. I don't know why the rear fog would work, however. Still, either the circuit "board" in the taillight is good, or it isn't. The grounds can get worse or better depending on humidity and so on. Also, the same is true for the connections. These are all very common problems.
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Chris Herbst, near Chicago, IL. 93 940, 91 240, 90 240, 88 740, 87 240