On my crew cab I dropped 3 nmo mounts near the back of the cab. I was able to drop the headliner just enough in the back and by the sides to see in there to make sure I wasn't going to hit a support and went for it.
I'd love to drop 1-2 more but I have the sunroof on mine so I couldn't go any further forward. I have put radios under the front seats but mine are all mounted under the rear pass side seat now in the crew cab.