On the '08 CTS, it seems that the outer door panels are a different material than the front fenders and rear quarter panels. The fender and quarter panels sure sound like metal, but the door panels sound like plastic. I had to prove they have iron in them with a refrigerator magnet, and yes the door panels hold the magnet. So what are they made of? Maybe a different grade of sheetmetal? Maybe a composite of metal and plastic? Anyone?