Ken,

If the head unit you removed doesn't have an antenna connection, then the tuner is in the trunk (right side between the hinge and the fender)and the amps are behind the rear seat. You will have to run speaker wires at least to the amps, an antenna cable, and possibly a signal wire to the antenna.

I don't know of any drop in in-dash CD players or wiring adapters to make an easy job of it... and I've looked.

One option is to get a portable CD player and connect it to your system by a cassette adapter (I've heard some MB head units don't take these, but maybe it's the later models) or an RF converter (~$20 at Radio Shack).

I have an RF changer in the trunk and it was pretty easy to install. I didn't touch anything in the dash.

Sixto
91 300SE
81 300SD