It looks like that may be the way to go, Crutchfield has this:

http://www.crutchfield.com/p_120702550/Mercedes-Receiver-Wire-Harness.html?search=120702550&ssi=0&tp=2977

which sounds like it would be a good adapter. They have the same model stereo as well, so I'll get that and they can walk me through it all.