If your car is like our '90 744, the bulb holders have differences in them that require them to be in specific locations.

Your mention of switching bulbs around sounds just like what I did when I had a bulb out.

I almost melted both tail lights when I switched the bulbs from left to right.

If you didn't switch them, could someone else have done it?

Careful attention to how each bulb holder fits and makes its connections will allow you to figure it out.

BTW the sockets are color coded, perhaps someone can tell you how.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '12 XC70, the first three are mine, heh, heh, 525,000 miles put on 10 bricks