Wires?

This is a picture of the 6-wire harness.

That is for the trailer wiring adapter. It looks like it might be disconnected, I would use a flashlight and get under the van to see if the main harness got unplugged, do the rear light work? If not, this will be the problem.

CARTH H HUNT JR. My name is Ernest and I'll be glad to assist you.

First, I'll need the specific model of your vehicle, as there are many options available which would determine the wiring harness used.

The 1500-3500 came in passenger or cargo, standard or extended. The 4500 only came in cutaway. Also let me know if you or a prior owner had any aftermarket accessories installed. As soon as you can get back to me with these details, I'll be glad to assist you further.

It's a GMC Savana 2500 passenger van. I didn't install anything new.

Seeing the picture, I agree with KEN L. that is the harness for hooking up a trailer hitch. Definitely hook it back up as it will flop around until the wires and connector get damaged.