Or hook your own truck to another trailer. Either way, it would help eliminate or confirm the truck as the problem.

 

I don't know. It doesn't make sense, unless you have some sort of non standard trailer lights or wiring. On every trailer I have looked at, the brake, turn, and flasher circuits are all one and the same. Heck, when I hook up, I just turn my flashers on to verify that my brake lights and turn signals work on the trailer.

I don't get it. Are the flashers working at full brilliance on the trailer?

 

Take a battery or battery charger over to the trailer with 2 pieces of wire hook the neg to a good CLEAN ground on the trailer then take the + hot lead to all the different circuits on the trailer plug OTHER than the ground and just a very short tap to check the brakes. I always do this before i tow something other than my own to see if they are wired correctly and it works great for trouble shooting.
If you don't get everything lit check for power at the sockets,bulb contacts and filaments then move on to the wiring it self if you need to.
It is just a simple process of elimination.
When i hook on to go i turn on the four ways and the parking lights do a walk around to see if everything is lit up then i am good to go.