2011 Chevy express 3500 with supreme Spartan service body rear brake lights not working

Unfortunately, you're dealing with non-factory components on a service body, and that in itself has its own wiring diagram independent of the Chevrolet chassis and body. Ideally, you'll be able to reference a wiring diagram for the added body, but the other thing that can be done is that you can trace the wiring back from the lamps - verifying there is no power at the lamp in the first place. There could be circuit protection (fuses) somewhere, but without reference information or hands on inspection it'd be impossible to know where to look.