Hi, my name is Marty. It is not uncommon for the main connector to the rear lamps to corrode or burn and cause a poor connection to the lamps. I would disconnect and inspect the multi-plug connection to the rear lamps and see if there is signs or poor contact. There are no replaceable fuses in the circuit. The circuit is internally fused in the rear signal acquisition module. I've also seen water get to the rear signal acquisition module and cause problems but I would inspect the lamp connectors first.
Please let me know if you need further assistance with this problem.