I'm not exactly sure how this works, but I believe it connect to the factory harness. Check out THIS
, Fifth Wheel Adapter Harness.
2000-2007 Dodge Ram Pickup Full Size Truck w/Factory 7-Way
You may have to drill a hole to mount it inside your bed or install it on an L
bracket, as Greg did with his.
Also, the 7' loop
below can extend from your factory 7 pin connector, at the rear bumper, to your fifth wheel 7 pin wiring harness.