Mirror heating troubleshooting (w211)

[rookie2012]

Hi,
CAR: E320 W211 2003 year, estate.

My passenger side mirror heating is not working.
Drivers side works fine.

Both sides electric adjustable motors works fine.

Also, they are non-folding.

I have removed glass from passenger side, and checked the two wires for 12v.
They have 12V coming to them, but the voltage drops to around 0,5v after some while. This was tested with around 12 degrees C outside temp, and drivers side mirror got very hot, whilst passenger side was still cool.

Any ideas how to continue and what could be causing the mirror not to heat up even though 12V coming?

Thanks

 

[television]

The glass can go open circuit, and this is normally the problem, you can test it on an ohmmeter