Foil-side (or other vapor barrier-side) on house insulation usually goes on the inside-of-the-house side. However, that rule changes depending on which air mass is the most likely to condense and which side you provide vapor escape (dependent on the climate in which you live).
In a car/truck, the foil side probably serves a different purpose than it would in a house. I'd look for single-sided padding of similar construction and see what the manufacturer suggests when used in a vehicle.
