I don't know how consistent MB has been between model years with their wire coding, but the wiring diagram I have for 1979 shows a red and black wire that carries power from the radio to the automatic antenna. A red wire, with a fuse in it, carries power to the radio and a brown wire attaches the radio to ground.
If you don't have a volt meter, I would suggest you go to an auto supply store and pick up a power indicator; probably a couple of bucks. You connect one lead of the indicator to ground and then probe the various wires with the other lead. If the indicator glows, you know you have power in that line. In the case above, for instance, (if it is right)a connection to the red wire should cause a glow when the key switch is turned on and there should be no glow if the key switch if off.
Hope this helps. Good luck. Let us know how you do.