The head unit is just a pushbutton unit with a cassette player in it. It does nothing but send data to the tuner unit in the trunk. The trunk unit is the actual radio that generates all of the audio with these multi-piece radio setups. You can pull audio from the trunk unit, but you cannot use the head unit by itself since it's basically just a pushbutton unit. See your other thread in the multimedia section, I gave you a more detailed reply there.
Becker Grand Prix 1432 Aftermarket Amplifier

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]