Skybase

So I've used my towing hitch for the first time the other day to bring home a small utility trailer, and all lights worked except the brake lights. I think part of the issue stems from the fact that the vehicle brake lights are a separate light cluster than the taillights, turn signals. The brake lights are of course higher. So in wiring surely these don't use the same wires unless they're directed to do so, probably in the rear SAM?
Anyhow noticing that there are only wires plus ground. (tail, left, right, ground)
Being that this is a factory install, is this a sign of the rear SAM unit having issues? When I'd first gotten the vehicle, the rear SAM was preventing the rear hatch from opening etc but I removed it and cleaned it all up and since, everything else has been working.
Hoping that this is something easily resolved. Surely it wouldn't just not work due to the wiring harness that is installed. (simple straight line, 4 wire unit)