Hmmm.

Seems like you have backup light power going into the light panel. We've had a ton of posts re. rear lights on sedans lately.

I'd take a close look at the path from the incoming connector at the rear lights' circuit "board" (flexible plastic, right?). One copper strip on the board is the ground. Anyway, see how the bulb holder contacts the conductors at the backup light location. Test the contact strip on the circuit board there. Keep looking. Somewhere the path isn't passing current.

Have a look at the contacts on bulb holder. Maybe bend out spring contacts onto the bulb so they connect for real. Maybe probe the back end of contacts in bulb holder (are they visible??) to look for voltage.

Be careful about bending out contacts where they meet the "circuit board". It's not too strong. I tore one of mine once (tore the plastic "board").
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Sven: '89 245, IPD sways, electric rad. fan conversion, 28+ mpg - auto tranny. 850 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).