Metal roofs and engine hoods will block GPS completely.

Mine (Garmin GPS16) is actually in my glovebox so any signal has to make it through a layer of dashboard, various metal brackets, various electrical wiring and an airbag. I do drive a Jeep so it does not have to go through a metal roof. It does take it a little longer to acquire sattelites (up to a couple of minutes) than if I set it on top of the dash, but it can generally aquire at least six sats with good signal strength.

Anyway, where you can place the antennae is going to depend on a variety of variables and may require some trial and error, but with a decent GPS I would be suprised if you could not get a usable signal from someplace within the vehicle.

-Mike