Trailer tow wiring issue

My dad's 96 Ram has got some wiring issues for the 7-pin plug. It used to work, but now it seems that only one or two of the leads have any juice on them, and the rest are completely dead. It's a 2500 4x4 V10 with factory trailer tow package.

Here's where it gets a bit weird. We have the service manual, so in an attempt to trace down the problem, we referenced the wiring diagrams, and found that the two plugs that are supposed to wire into the 7-pin plug are just dangling free, exposed to the elements, and corroded beyond use behind the rear bumper. But, there is wiring that goes into the plug, all of which is still secure. My dad is second owner on it, so we though that maybe the first owner had messed with it, but despite the fact that the correct plugs don't appear to be in use, the wiring that is in active use looks fairly factory as well, doesn't really appear to be manually spliced in or anything.

Anyway, neither the used or unused wiring for the 7-pin plug seems to be working correctly, and we're not sure where to start looking since the wiring doesn't seem to match the service manual. Again, the plug used to work just fine, but it doesn't get used much for towing, so we have no idea when it stopped working. Anyone know of any common issues with the trailer tow wiring for these trucks? The local dealer would have us believe that we have to install a whole new rear wiring harness, and we're really hoping it doesn't come to that.

Well sound like the road salt/mud has taken its toll on the wiring back there. You going to either end up splitting the loom open and cut back to clean wire and adding back on a new plug or replacing the wiring loom completely.

I think factory was 10 pin. I had two sets of wires going into it.

I cut it and spliced in a 4 pin.