I'm copying tbrudder's post from 2003. It seems he found and detailed what I now need to do to make this work.
"Basically, it would seem that even though my truck didn't have the trailer tow package (just a Mopar hitch, I'm thinking), the front end of the wiring harness was still in place. I'm going to go so far as to assume that all trucks are wired like this. Checking my PDC, the trailer fuse (40A, IIRC) and relay were in place. Using what I read here, and some info from the dodgeram.org site, I found that the wires for the trailer running lights, electric brake, and B+ ran down to one of two connectors (C128 & C129, I believe) located at the very back-rear of the engine compartment on the driver's side, just about opposite from where your feet are on the inside. The rear part of the harness does not carry these to the back. I just tapped the wires I needed on the front side of the connector (I believe brake was blue, running lights orange/red, can't remember B+--maybe black) and ran wires along the wire loom back to the connector, and re-wired the trailer plug. "
Thank you Mr. tbrudder