It sounds like the older mechanical system, both methods apply (just to complicate the issue!);
The +12 volts is switched to the fan motor by a relay, when the ignition is turned on.
The speed is controlled by the resistors on the low or earthy side of the motor.
With the first switch position being Off where no resistor is connected.
The relay is G1-03
The resistors are G1-05
and the switch is G1-06
The electronic "Climate Control" version is different however ...