...I had someone suggest hanging a simple 1/4 wave from the roof panel on the inside, but that would put the antenna about 10 inches from my head! I've lost enough hair over the years and with 50+ watts, I don't want to cook what I have left...
Then don't run 50 watts. If you're around town talking through a local repeater then you
shouldn't need that much power. I rarely run my mobile rigs above 5 or 10 watts unless
I'm out in among the hills where I'm shielded from the repeater.
But just about anywhere on the car that is fiberglass and you can get access to it you
can stick a dipole or quarter wave vertical with two or more radials using burglar alarm
copper tape or thin wires attached to the inside of the body with tape (for temporary
testing), glue, etc. You can bend the top of the vertical wire if you don't have quite
enough height. You just have to look at your car and see where it will work.
I also saw an article where someone used a wire hanging down from the bottom of
the car as his antenna. There are lots of possibilities, but they will have to be
custom designed for your specific situation.