With it being a 129 model I have never seen one without this option. A good tech can take out the center console and locate the plug that was used for this. They have them even if the receptical isn't there.

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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