If they replaced a 'module', it was the TIPM (Totally Integrated Power Module) - AKA Fuse Box.
That's a $1000 module!
The reverse light wiring is as simple as it gets....switch sends signal to TIPM which sends power to the 2 lights and the 7-way trailer connector. That's it...nothing else in the loop.
If they replaced the TIPM last time, I'm willing to bet they will have it for a while to trace the wiring.