Check this site out that's a friend of mine:
Disguised undercover covert license plate antenna,fender,cowel,bumper antennas
The license plate antenna is a real compromise but if its for undercover work in a large public radio system that is designed to work well with hand helds, (voter receivers, etc) it should work ok. A friend installed one for general purpose amateur/GMRS use and it was very disappointing.
I've made and used simple 1/4 wave verticals from thin wire taped to a window with two ground radials along the lower window and it works surprisingly well but 110 watts inside a vehicle would be a health hazard.
prcguy
prcguy