'81 300D - No Backup Lights
 

The bulbs and fuse are fine... any ideas what to check next?

If you have no lights when reversing while bulbs and fuses are OK it may be that the electrical connector on transmission (which actuates the circuit to bulbs when the tranny is in reverse) is faulty. This happened to my 83 230E, the connector and connector housing on the tranny were rotten by time and exposure to the elements.

OK, I fixed the problem - it turned out to be the fuse not being seated correctly. The fuse was fine which threw me off initially. I actually did get under the car and test the neutral safety switch (all in good order) before figuring this out. All the wiring, bulbs, etc. all was working. It had to be power. So I just reseated the fuse and voila. This is the second problem that turned out to be a poorly seated fuse - is this a typical problem for the 123?