Corvette distributor sheilding was installed because the Corvette body's firewall being fiberglass didn't isolate the radio from ignition noise, it had nothing to do with reception whatsoever.George
That's a rather skewed way of looking at it!!! If radio noise and other static does not "affect reception," I don't know what does????
Just raising the metal hood on most any car/ pickup "back in" the AM days would result in a terrible noise from the speaker. The GM "good idea" of a windshield antenna later didn't help, either.
So far as a ground plane, that is completely dependent on antenna design. Not all antennas need a ground plane, as some Marine VHF/ UHF antennas for boats are what is called a '1/2 wave' design, and can be mounted anywhere on a glass/ wood boat.
In amateur radio, the Cushcraft R3/ R5 multiband vertical antennas are such a design, and can be operated without a ground plane
