Most people tap power off of the 12v socket. I disassembled the 12v car power adapter that came with my Roady and soldered wires onto the adapter and the other end of the wires soldered onto the backside of the 12v socket on the truck. Works great.
The sound quality off of an FM modulator or transmitter is awful. You'll want to direct connect the XM2GO into the headunit if possible. Not sure what setup you have in an 05.