Personally I do not think that the car is pre-wired with a coaxial wire for the cell phone, but I am not sure on an US model. The power Hirschmann antenna for radio and cell phone is the best there is; doe the connector that you have found have an additional remote wire to control the antenna? (to get the antenna up for the phone while the radio is off)

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)