sonar

Does anyone here have info about aftermarket nav/sat radio systems that fit an SLK? That's about the only things I think this car will need. (I've read SLK32.com's howto, but am looking at other ideas.) Also, can one t off the gps antenna output that comes with the car for a nav system? Also have the 6 cd changer, standard with the Se if that affects anything.
Thanks