trailer harness Q's

[Longhorn Man]

OK, now, keep in mind, my burb isn't here right now.
On the trucks, the harness for the tail lights is under the truck. I had made a plug in piece that plugs in-line with the harness.
Flipping through the assembly manual I have sitting in my library (the crapper) I noticed the tail light harness runs up in the roof.
Is there an easy way to run the trailer plug? All I need is a 4 flat.
I considered plugging in at the firewall where the harness comes out... but again, I am thinking truck terms... I don't *THINK* there is even a plug on the firewall.
So does anyone have any ideas?
Anyone done this?
Anyone UN-done this on there rig?
I'll need tail lights if I pull the pop up camper to the meet this summer.