Take a look at this site -- info on lots of Becker Radios!
http://http://www.beckerautosound.com/754/index.html (back panel diagram has info on what each post does)
http://http://www.beckerautosound.com/mercedes_year_model_1981-1990.html
When I replaced my radio (there was an aftermarket radio installed) I may have bought new speaker wires and a wiring harness from here. I found and installed a Becker 1480 as the look is similar but the antitheft sytem allows for a code to be entered rather than needing the reset device at the dealership (not sure if the dealers even have them anymore)