Just separate all the grounds... if you fish the wiring out of the center console you can see that all the speakers have a + and - wire running to them... if you get it all fished out you will see what I mean... you should have two red wires and two green wires and then there will be four black wires total...
This is all assuming you have four speakers in the car already....
(This is how my '83 300D was... I just did it on Friday so it's still kind of fresh in my mind
)