[UPDATED] One of the most frequently requested modifications for the 124 Spider in the UK is to be able to have heated wing mirrors. Read on to see if it's possible...

These are available in the US and in other territories, but for some reason weren't offered on any UK models.

Having done quite a bit of research, there's a few things to note:

1) There are multiple different versions of wing mirror (external mirror) available in the US. However, most of these aren't available in Europe, let alone the UK.

2) Luckily, it seems that the Fiat & Mazda wing mirror parts are identical, so whereas the Fiat parts cost over $400, the Mazda equivalents are about $200.

3) Without some dis-assembly work, there's no way of knowing what wiring exists in UK cars. Some others in the US have reported being able to fit heated mirror bodies into cars that didn't come with them without issue, but the wiring for auto-dimming and blind spot detection are likely to be different (blind spot detection utilises a separate control box).

With all these things to consider, and the vast number of unknowns with UK cars, this might not be the easiest project to complete, but I'm going to stick my neck out and say it is possible.

[UPDATE 14th April]
Having done some more research on UK parts available for both the Mazda & Fiat versions of the car, it seems that *all cars* are pre-wired for heated wing mirrors. The same wires will also allow the fitment (although other extra parts may be required) of the Blind Spot Monitoring system.

Physically checking both Fiat wiring looms that pre-exist in cars, plus the exact pinouts and colours of brand new Mazda heated mirror bodies also confirm the above to be the case.

The next step of this investigation is to acquire a Mazda heated mirror body & glass (at a rough cost of £150, so £300 for both sides) and test it out on a Fiat vehicle.
I'm considering funding options available to me to enable this to happen.

[UPDATE 31st May]
Yet more research shows that whilst the correct wiring for heated mirrors is present at the plug in the door loom; the supposed fuse location for heated mirrors is NOT populated on UK cars (at least on Abarth models). This means there may be no way of making the "live" wire of the heated mirror loom actually carry any current. At the moment I'm hoping that I can piggy-back this wire to the "live" feed to the mirror switch pack, which should be live all the time. However, lots more reading of wiring diagrams (and probably a fair bit more dismantling) needs to be done to confirm this theory.