I am the owner of a 1972 240z and I can't get the blower fan to turn on what could be the problem.

Check the relevant fuse for the blower motor.

Have you tried it on all speeds?

There is a dropping resistor inside the housing that provides the various speed settings.

Check the wiring going TO the motor. You might find the IN-LINE fuse going to it being the culprit as Graham mentions. The "AC" fuse is Accessory.

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