Disconnected the fan motor and ran it directly from an external battery - pulling 2A on low setting and 4A on high - no obvious issues. Tried again using the battery in the car, with the car ground wire connected to the battery, and suddenly current increases and the motor smokes. Quick test confirms that the green/yellow and green/blue wires are shorting to the metal casing.
On dismantling the motor, I found this inside:
The wires connect to the motor brushes with around 1cm exposed. The brushes sit in the plastic plate, pushed onto the motor by springs. The plastic base plate had broken, allowing the live wire to move and intermittently touch the outer casing causing a short.
Putting this here for info, (1) to help anyone else chasing a similar problem in the future, and (2) to see if anyone else has had similar issues with aftermarket motors? I have contacted MR to see if they will provide a replacement under warranty.
Not helped by the fact that every time I turned on the ignition to test this morning, my rear carb overflowed as the float has sunk...
