I'm pretty sure you can use your existing shell. The heater is just a little block that sits on the back of the mirror (Thats the way the continental is anyways). If I am right...the wires might even be there already too.
First off. Thanks for everyone who replied. Frank, I am indeed going to upgrade to heated mirrors since I live in WI. tmonster, if you do go to the bone yard, please keep your eyes on some Gen III heated mirrors. I am in the market for some. If you see some, please give me a price. Are the heated mirrors worth it?