Where did you run your antenna wire to the back?

My guess is your antenna wire is picking up interference as its running parallel to the wire that turns on the fuel pump.

If you ran it along the bottom side of the truck, try routing it along the top of the truck, inside of course, like where the headliner and side of the truck meet.

I'd also just do a quick test with the wire loose and way from everything to see if it goes away first before re-routing....