I don't think Mercedes used a floating ground for their speakers. If you find you don't have one positive and one negative wire for each speaker, you WILL have to run new ones. I doubt it though. By the way, I figured that you knew how to take the ashtray out already since you were talking about replacing the radio.
Be careful putting the radio in, I found the wires in the back were getting a little pinched since the clearance was so small once in place. Of course, I have 3 sets of RCAs and a DC-DC converter hanging off mine.