HI,
I just went to get my 89 420 SEL inspected for my yearly donation to Louisiana, and found both backup lamps are dead. I replaced both and found the circuit is dead for only the two backup lamps. The other rear lamps are fine, tail, turn, and stop.
The fuse is for a multitude of things that are all still working, like the AC etc.

Any idea about where to start?

I forgot to say that there are NO dash lights to tell me the backup lamp do not work.

The most likely culprit is the Neutral Safety Switch (NSS), located on the side of the transmission. Its two functions are to prevent the starter from being activated other than in park or neutral, and to turn on the reverse lights. Not too expensive, and not too difficult to change out.

There's always a chance you have a damaged wire somewhere, but NSS failures are not unheard of, so starting there makes sense.