What Andy said.
The umbilical is short and wire gauge isn't that critical except maybe for the charge line and ground if you're trying to get the battery in the traylah to charge while under way.
So, the turn signals, and backup lights, for instance, any gauge is fine, you could run 20 gauge and never notice any problems. Brakes, for a tandem axle, 14 gauge though 16 will probably work, triple axle, 12 or 14 gauge. Tail and marker, if you have LEDs, 16 gauge is enough, 14 otherwise.