I found this out in cyberworld..... ref 2003 Duramax
"If you're truck did not come with the FW wiring option (it will be a bundled up cable at the front edge of the pickup box, next to the frame rail) you can purchase that wiring option from the dealer. It's plug and play. It actually plugs into the trailer wiring harness just above the spare tire. It's really long.
A 2" metal hole saw works nicely for the Bargman 7 pin socket. I mounted mine just in front of the wheel well at about the two o'clock position (as viewed from the passenger side). There's a nice flat spot there. After I installed the socket on the end of the wiring harness, I completely filled the back of it with marine silicone. I've replaced too many of these things with the wires totally corroded."
Somewhere around here I've got the actual dealer part number for that FW harness if your truck did not come with the "UY2" option. Maverick clued me in that it was "plug and play". I'd never wrung out the cable as it disappears above the frame and gas tank and then goes back to the rear of the box.
Some folks just plug into the connector by the receiver hitch. I don't particularly care for that method, but it works.
I just got the 5th wheel harness, it has three ends on it. One end has bare wires for the bargman plug, I understand that. The other side of the wiring harness has a split with one male on one side and one female on the other. Can you help me understand where I'm suppose to plug it in at, or do I have the wrong harness.
unplug the harness from the back of the trailer plug on the hitch and plug the new harness into the oem plug, then the truck harness and the trailer harness will plug into each other, your essentially T ing in with the fifth wheel harness
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