Left Rear turn single not working also the Left turn single on trailer tow plug I noted that the two wire tested both not working I think the Problem is in front of the truuck
If a short circuit is detected by the front control module a fault will be set and that circuit will be shut off. After the fault is erased with a scan tool the circuit will be restored. You might try disconnecting the battery for a moment to see if that erases the fault.
In most cases the “short” occurred in the trailer, not in the truck.