The manual says you cannot use the existing XM antenna.
Here's the unit I got on Crutchfield:
http://www.crutchfield.com/S-XzpsIOR...m1&i=607SIRGM1
If you have XM already, then it is one wire to connect. If you don't have XM (like me), you need to connect power and a passthrough cable. If you don't mind pulling apart your dash, its an easy installation as long as you can find the power! (Orange +, Black -)