The problem is that the buttons are multifunctional, they're also used for things like the phone, mileage, odo, clock settings and such. My guess is that they are not directly linked to the headunit but that there's a ecu inbetween somewhere.
I'll take the headunit out and see if I can find a signal on its wires.


You didn't specify model and year, But I've got a feeling that you are out of luck.

Recently (depending on the year and model) Mercedes moved to a fiber optic system for their electronics. There aren't any aftermarket adapters for it out there yet.

In my '02 230K, the head unit itself has power and optical inputs. That's it. The head unit plugs into Mercedes custom d2b optical ring and the amp for the speakers plugs in there as well. The CD changer plugs into the same ring, as well as the phone ($1800) option. All of the controls link in, with the multifuntion display. It's not going to be a simple tap in.

It's tough enough cookie, that I'm putting mine in via FM modulation until an adapter becomes available.