On my fathers van -- (a Chevy Lumina APV) the entire thing is fiberglass. Not good for a mag-mount antenna. But in true "ham" fashion I came up with an idea. I took a piece of scrap metal, and put suction cups on the bottom. I also took wire coat hangers, cut them down to 13-14 inches long, and attached them to the bottom for the ground plane. It worked well, until it blew off the fan going down the highway.
I rebuilt it, and made some refinements. I no longer had the scarp metal that I used the first time. So I took a piece of sheet metal that would be used to plug a heating and cooling vent. I bent the edges down flat, and added the suction cups. I then again took the wire from coat hangers, cut them about 13 or 14 inches, and used JB Weld to attach it (I tried to solder, but that did not work, and I do not have access to welding equipment). It works fine, and is still on top of the van (have not pulled it off yet, even though my father broke off one of the radials). We have some other ideas, such as having a tie down to keep it from flying off. This is also a rather cheep way that is not permanent.
For right now all we use it for is my HT (my father is also a ham). He does not what to do anything though to his van, but this option seems to work. I have no idea how it would work with a higher output radio.
