When I was in high school (around 1970) a friend took me to a vacant lot that had three old cars (early 50's sedans) that had their interiors burned out. They were sitting on their roofs. I found a few Wheaties lying on the ceiling of one of the cars.

A few years ago I was at an old mine site in Arizona and there was a 50's vehicle there, on its side. I was able to push it over and detect where it had been laying -- hoping some coins had fallen out the windows. No luck but I did find a few items (don't recall what) that had been lost under the seats, etc. So I coulda found a coin....

I didn't answer Sven's original question. A shaker screen like archaelogists use? (Probably just get clogged up with rusty chunks.) A big electromagnet? (Easier said than done.) Sometimes the best treasure finder isn't a metal detector. Just ask bottle collectors.