Volvo Repair: Break light puzzle on Volvo 760 GLE 1988, volvo 760 gle, advanced auto parts

Question
the puzzle is: initially the only problem appeared to be the driver side break light was bad while the passenger side break light worked fine.

Next, I replaced the bad driver side bulb with a Advanced Auto Parts replacement bulb resulting in no cnange.

Next I replaced the light assembly on the driver side.

This is where the puzzle kicks in.

At this point the passenger side break light started  working while now the passenger side that was working is now not working.

Next I replaced the bulb in the passenger side with no change.

Answer
John,
I don't think the bulbs are the problem, but rather weak grounds. The bulb housing has connectors with 2 wires to the bulb. You can trace that one is hot wire to the center contact of the bulb- the other is ground that contacts the outer metallic part where it plugs in to the housing. You can check the hot wire with a test light to a good ground on the one that is not currently working. If the test light burns when you know you have a good ground established, then you know that the bulb is not getting a good ground when it is all assembled, perhaps due to oxidation. Try cleaning the contact points where the bulb meets the housing, where the wire plugs into connector, etc.