Heated Mirror Fault

[Westyfield2]

One of the heated door mirrors on my C7 Avant has stopped working. Other is working perfectly fine.
Did a VCDS scan and it came up with:

1 Fault Found:
00920 - Heated Outside Mirrors; Driver Side (Z4) / Passenger Side (Z5)
012 - Electrical Fault in Circuit
Freeze Frame:
Fault Status: 01101100
Fault Priority: 3
Fault Frequency: 1
Reset counter: 135
Mileage: 69755 km
Time Indication: 0
Date: 2016.11.18
Time: 18:11:24

I've popped the Mirror glass off and stuck a multimeter on the cables.
On the working mirror it's what you'd expect; 0v when off and 12v when turned on.
On the broken mirror it's a constant 4.5v, with no change when the heater is turned off or on.


So, any suggestions?
My next guess is to trace the cable back further to see if it's getting 12v at an earlier connection, and my guesses on the break location would be where the mirror bolts on to the door (in the folding hinge), or where the door bolts on to the car body.

[Harley8588]

Have you got a circuit across the mirror element? Switch to Ohms and see if have a resistance reading between the connections. If you have anything in the Mega Ohms range your element has gone open circuit.

If you have 12v at the spade connector when you turn them on then there's no point looking further back in the circuit

[Alan02]

The 4.5v could be a reference voltage if the element is open circuit.