There should be 2 power wires on the rear of the radio. One holds the memory while the other turns the power off with the key.
Power is usually red with a stripe
Keyed power is usually black with yellow stripe.
Ground is brown.

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Benzmac:
1981 280GE SWB
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM