TBH if the ariel cables are made up properly then the earth plane shouldn't be a problem, mine isn't. I know the ally roof doesn't help but there is enough steel in the frame to make a good earth.
I have my guttter mount above the rear door and I managed to get a SWR above 2 (or is that below, better than anyway) though when I check it now I don't get a reading, I think the SWR meter has hit the dashboard one too many times now.