GM trailer light hookup nightmare.

[Biker1mike]

Maybe you can take a look at the wiring diagram for the van to see if something looks off. If all else fails, it might be worth bringing it to a mechanic to have a look and see what they can recommend. It's possible that the age of the current trailer is making it difficult to troubleshoot.

A new member error. Check the post date. This was a couple years ago.
It happens.