I assume you mean the fan INSIDE the car, not the one in front of the car?
Do you have Automatic Temperature Control (digital display) or manual air conditioning?
In a 75 it's probably ATC (Automatic Temperature Control)
, so check the fuse link 10 in the underbonnet fuse box. The brown blue wire then takes power to the blower itself, check this as suggested above but it will only be live when the ignition is on. If theres no power it could be the fuselink, or a relay in the passenger fusebox. It could be a wire from the ATC
controller to the relay. This is all unlikely, but check it first.
If that's OK, check the voltage out of the other side on the plain brown wire. If this is all OK, the main supply and connections to the blower are OK. The brown wire connects to the power tranisitor, which also has a big black ground wire that should be checked.
There are two control wires from the tranistor module to the control unit itself. It's unlikely to be these, but worth checking anyway that the wires are still plugged in.
Finally if none of that, it's worth checking the fuses to the ATC
control. These are fuses 4 11 and 33 in the passenger if you have the older grey fusebox, 11, 27 and 34 on the newer red boxes.