05 columbia no reverse light

05 Freightliner columbia that has no reverse light. Light and pigtail were replaced before I got to it. Checked power and only have 10v but not enough to power up any kind of light. Followed harness along frame and have 10v at a connector halfway back. Had the original circuit breaker installed in the fuse box, replaced with fuse and have good 12v power at fuse. I cannot find any diagram that shows me the wire coming out of the fuse box or at the bulkhead connector at the firewall. Anyone help?

 

Is the reverse switch before or after the back up light fuse inside the cab? I cant find any diagram that shows where in the circuit it actually is.

Ground at the light it good, used it to light up a differnt light to test it. Eith the harness disconnected going to the back i still dont have good voltage