I agree with LILRED66, except I would describe it as the foil side is towards the inside (as LILRED66 said, as inside a house) - this is completely correct, even for the ceiling/roof of the truck (where "up" might be interpreted incorrectly).
Make sure the mat is completely glued to the outer surface and make sure all seams are sealed (so that moisture cannot get between the mat and the metal).
Tim
