Mainsail - I just installed a NMO trunk mount on the passenger side of my M1009. I plan on putting a dual band antenna on it so I can use my Yaesu FT-7800R dual band in the truck. My concern is signal radiation being sent straight into the cab.
The Chevy roof is a double layer of steel. Run the coax up through the rear pillars (where the shoulder belts mount) and mount your antenna center of cab about 4-6 inches from the back of the cab. Don't go too far forward - the two layers are far apart at the rear, but get closer together as you go forward. You'll run out of room.
Now you've got those two layers of steel between you and the radiating element. Between that and the antenna pattern (most of the energy is on the horizon) you'll have very little exposure. In fact, I'm pretty sure this will give you less exposure than Mainsail's mount. (Of course, his is mil-spec, so it will look cooler than a roof mount, so there's that to consider.
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Hook that up with some RG-142 and you should be good to go.
HINT: A length of standard electricians fish tape works well for fishing wires through ceilings and pillars. When I was installing (business radios and cellular phones, back when they were installed in cars) for a living, I had several lengths in my tool kit. I'd heat the ends and bend them into an eye, then braze them like that. I'd fill the trailing edge where the eye comes around and meets the rest of the tape, then file it smooth. Makes for a very smooth pull with nothing to catch on edges.
Once you have your hole drilled, run the fish tape into the hole and back towards the pillar. It should drop right down where you can grab it, and pull your coax through. I lost count of how many cellphones and radios I put in mid-80's Chevy pickups!
By the way, I've been following the issue of human exposure to RF closely since the mid 80s, reading everything I can find on the subject, including the scare mongers. There have been plenty of studies, going back to WWII. As long as I am not getting levels that cause actual heating, I'm not worried about it.