This might be another dumb question but does the GPS antenna need to placed with the top facing up?

I was thinking of sticking the metal sheet to the inside of the windshield behind the rear view mirror, which would point the magnetic side/bottom facing the sky. Would the signal still be strong enough?

I've seen a few people do that with no problems. Try something non-permanent like tape to stick it up there and try it out for a few days to see if it is OK.