Several good ideas here, I'll tell you what I did. Mine's on my rear bumper. I knocked the aluminum plug out of the hole in the driver side floor, probably similar to what willysrule did. You can see it from underneath but not from inside the cab. I ground me a groove in it for the coax, ran the coax through the hole and under the carpet, replaced the plug with lots of silicone, and alls well. I did have to trim some type of material around the plug inside the cab to get the plug out without damaging it.
Scott