Rear bumper, I'd say. Problem is you gotta have a way of keeping it tied down otherwise it'll fold over and stick out the back of the rig. I tried whip antennas on magnet mounts on the roof, but low lying anything of course knocks them over. I ordered I think a 5 or 6 inch stubby antenna and have been using it the past couple years on my 88 Bronco 2 and it seems to work fine for what little I use my radio for, which here the past year has been not at all because I'm no longer getting any audio (cheap Chinese crap radio) so it needs replaced, and haven't bothered because I haven't really used the radio prior to that enough to care if it works anymore or not.
You can see it on the cab roof in the photo, its pretty small...nice thing with them you can mount a bunch of them up there and not have to worry too much about hitting them on anything.