Originally Posted by redhawk
When my trailer is plugged in to 110 volt the backup lights go on. When un plugged and hooked to the truck the backups go on , On trailer, and truck! I have no batteries installed at the present could this cause the fault.
No, the battery is not the problem.
When you feed power to the trailer, regardless of source, your supplying power to the back up light circuit.
Pull the cover off of the 12 volt
cable and look for a strand of wire from the backup light circuit touching the charge line, or the reverse of that.