Welcome! I was stationed in Furth, the closest I came to Munster was Bremerhaven, and Cassel. I really enjoyed my stay in your beautiful country.

Jimc2002 is right. Start at the bulbs and test them. Then go to the backup light switch and see if it is getting 12 volts. If so then follow the wiring back to the rear harness. Sometimes people work on the transmission or controls, and forget to hook up the backup lights. Worst case I have seen is when a manual trans has been swapped in, and no switch exists because the factory switch is hard to find. Then the fix is to rig a toggle or push button switch to be activated by the driver when going in reverse. That may or may not pass inspection. Do they still require you to carry spare gas at all times?