Steen, no offense, but I'm not sure what the intent was with this suggestion. As we've noted, a roof mounted intake would benefit a race car for cabin venting/cooling - as Woody and Derek have pointed out - or as an intake for mid mounted engines.

For a normal road car, this mod makes little to no sense since it's intent is undefined. If it's for cabin cooling, just remember that you are now going to have to deal with an opening in the roof of your car where water can enter. As an engine air intake, as I described earlier, this is a troublesome issue with the routing of the air back to the engine and the same water management issues still exist. Additionally, I'm sure someone here could tell you the negative effects on drag as well.