turn signal - trailer problem

Have a problem with the left turn signal on a '04 ram 1500 when hooking up a trailer.
The signal light works on the truck but does not when plugging the trailer into the jack in the bumper or the plug-in in the harness in back of the bumper.
The problem is not a fuse problem; it is not a problem with the trailer lights.
I do not have a wiring diagram for the truck so I can't really trace it.
My Big Tex trailer dealer does no have a solution; I've talked to the Dodge dealer - he wants to go exploring @$100+ per hour.
Has anyone else had a problem like this?
Johnson in Tucson

Do the brake lights work on the truck when plugged in to the trailer wiring?

So the turn signals don't work at all, including the instrument panel indicators & front turn signals when plugged to the trailer wiring?

$100/hr for "exploring"? Who does this prick think he is, a neurosurgeon? I don't know what the cause of your issue is, but I do know that he's charging too much to try and find it, plus who knows how many hours it will allegedly take to find the problem.
My advice; shop around for a better price, because that is pretty steep for some electrical diagnostics.

I bet if you look around on your local craigslist you can find a backyard mechanic that will look into it for a lot cheaper.

I'm not even an electrical expert and I could probably get onto the right track with just a test light.