If your '74 230 is like my '72 W114 250, your heater and AC use 2 separate fans. Your AC thermostat is connected, via vacuum hoses, to a vacuum operated, blower change-over switch, located above the accelerator pedal.
Does your fan switch still operate the heater fan OK when your AC thermostat knob is turned all the way off? If so, it's probably not the fan switch.

I agree with Steve, you should get a wiring diagram.

If you decide to replace the fan switch, you should be able to unscrew the heater/fan control panel and pull it out far enough to reach the fan switch.

Happy Motoring, Mark

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DrDKW