You may want to check the tail light assembly. There are copper traces (like a circuit board) on the assembly that need to make contact with the metal prongs of the bulb carrier. If needed, carefully clean the traces. Also make sure the plastic of the assembly hasn’t cracked or warped causing poor or no contact.

I had this problem and got tired of dealing with it so I ran a wire directly from the appropriate wire at the connecter of the assembly to the appropriate contact of the bulb carrier - solved the problem.


Michael
Melbourne, Fla

1985 740 460k (original motor)
1988 780 Coupe V6 190k
1989 780 Coupe V6 149k (parts car)
1992 745 198k