Your Becker Grand Prix will be a 2-channel headunit. Many of the pre-1995 Mercedes-Benz came with 2-channel headunits but they came with 4 speakers. They wired them using a fader wheel in the centre console. Search for 'fader wheel' and you'll find more on this subject in these forums.

You'd need to buy the fader from a salvage yard if you want the original one ... electronics stores usually sell fader controls .. it depends on what you want.

It requires quite a bit of work to get the wiring done. The wiring come from the headunit, and then under the console, and then to the fader. Then it's split into four signals for four speakers. The factory wired them using a common ground.

Hope this helps.