If you follow the board layout it looks like power is on pin 1 on the potentiometer header. So pin 2 shoud be the wiper. The other "ground" But is not really ground.
A NO switch between 2 and 3 should give full speed when turned on.
Does your meter have say a 200k ohm range? The picture shows...
The 7812 only powers the circuit not the motor. If you look at the 8 pin ic it is probably a ne555. The P75NF75 is rated at 80 amps. This powers the motor. With the small heatsink and small circuit traces it cannot source enough current. If it is a ne555 looks like it drives the gate of the...