You cannot separate the head unit and the tuner unit in the trunk. The head unit is only the tape deck and the pushbutton controls. The actual radio and audio generation circuitry is in the tuner unit in the trunk. The head unit will not work without the cable connected to the tuner unit.

I did more or less what you want to do by feeding the speaker output ports on the tuner unit into a line-level converter to feed into a power amp. Be aware that the sound quality is not great doing it this way, the tuner uses some weird equalization that boosts mids and attenuates treble/bass. If you run through an equalizer it may be ok, but I hit the trash with mine and went full aftermarket stereo and haven't looked back.

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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]