I have done a couple of these...What works for me is the universal antenna that sticks to the inside of the window and looks like a small stingray.
When doing this, it is possible to connect to the radio phone input to mute the radio when you have a call coming in. (it looks cool for it to say PHONE toooo..) I think it is a yellow or Yellow/black wire on the back of the radio...
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS