I am now a self proclaimed flex board/tail light/brake light/lamp socket expert. I hope to be an expert with the reverse lights next. I think you are shorted to the ground. Pull out both of the tail lights and take a good look at the flex circuit boards. One of my boards had a spot that was shorted and burned out, hidden by the gasket web. Another area had the insulation worn through by so many turns of the socket that may have caused the short and other issues. It seems that the ckt board will burn out like a fused link before the fuse blows. In addition, for the three prong brake light sockets, white on the right and black on the left ONLY if the sockets say "Volvo". The PO put in after market white on the right which you would think would be fine and good but it should have been molded in black for the left side. That one took some head scratching. Make sure you have all the grounds, (three on my 1992?) connected with star lock washers.

One more thing I did, I clamped some 12" long wires to the battery plus and negative and checked every socket with the lamps in them, jiggled the lamps and checked for good contact. I found a few with weak contacts and the original Volvo white with no ground, which just needed a minor bend adjustments. (If you test using 12V from the battery, make sure you have a holder for the end of the positive side wire.)

First thing to check is the tail lights. The lower outer bulb holders are the only ones holding a dual-filament bulb. They have three metal tabs touching the printed circuit.

BUT - the holder on the right MUST be white, on the left MUST be black. If they are reversed, all kinds of weird things happen, like what you are getting. Backfeeding through a ground, etc. Besides, of the things that CAN go bad, this one is easy to check and fix.

Only difficulty (minor PITA) is that to remove the cover from the LH light you have to take out the spare tire.

Good Luck,

Bob

:>)