Side mirror heating

A long time ago, while sitting at a stop light, a bus tried to climb onto my Coupe.
Fast forward by 7 years, a full respray and 5mm of freezing rain, my side mirror fell off.
After getting a replacement set of mirrors, I am not sure if the heating element works and I don't know how to measure it.
Does anyone know what values should the heating element have and when the car sends 12v to the mirror?
I measured 12v once in the wires with the ignition on, but could not replicate it again.
I really don't want to dismantle my other mirror to check for volts or ohms.
Car is a 16vt LHD.

On the 20v Turbo I think the heated mirrors only come on with the Max Defrost setting on aircon ( what a great feature by the way, why don't all cars have this?)

There is no direct switch for the heaters - may be different on 16v but hope this helps

I was under the impression they come on with the heated rear screen??

They do indeed come on with the heated screen.

The 20vts are supposed to have heated mirrors.
However noticed recently mine don’t seem to do anything anymore!!
Checked 12v feed and there is resistance in the wing mirror element and everything seems ok but alas condensation just sits there!!! Can they wear out? Do others notice a change to their mirrors when they turn on the demister on these damp mornings?
Puzzled!

I've noticed the same thing on my Plus, if the mirror demisters did come on with the HRW, they didn't work. I also noticed that the HRW didn't turn itself off after 5 minutes or whatever period that it used to turn off on my Porto 20vt, maybe the heating/AC ECU was programmed differently on later cars?

Think it is circa 20mins for the HRW and mirrors until auto off