Lots of radio Hams use this method George and they seem to manage alright with them or a roof mounted magnetic base.
The latter yes as the steel roof forms an excellent ground plane to allow a stable impedance to radio. Ground planes for single ended vertical whip aerials can be simulated with horizontal or downward facing quarter wavelength spikes at base of whip, radially placed.
I guess a rear bumper with whip mounted centrally may provide some ground plane effect but wouldn't guarantee it. I guess it's form over function although I personally don't have an issue with drilling holes in a car that is held together with factory made holes

. If you rarely use the radio why worry though

Richard (M0DSU)