UPDATED 03/01/21: One of the most frequently requested modifications for the 124 Spider in the UK is to be able to have heated wing mirrors. Good news - 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, the Fiat & Mazda wing mirror parts are identical, so whereas the Fiat parts cost over £300, the Mazda equivalents are about £100.
3) In the UK, it is easy to get hold of heated mirror glass designed for the Mazda MX-5.
4) For whatever reason, Fiat & Maxda chose to exclude most of the necessary heated mirror wiring from UK cars.
All these factors mean this isn't the easiest of retrofits; but there's nothing which can't be overcome. However, there is a lot of work to be done.
Starting with the schematic (this is from an MX-5, but the wiring is identical), we can see that the heated mirror option is part of the heated rear window (or window defogger, as Mazda refer to it) system.
Modifying this diagram to show what exists and what doesn't within the 124 Spider, we end up needing all the highlihted parts:
So to summarise, the items we need are:
1) A 12V power feed from the rear window defogger relay
2) A new fuse connection within the cabin filter box, containing a 7.5A fuse
3) Wires from the fuse connection in number 2) above to both the left and right hand doors
4) A connection within the door mirror bodies to the heated glass
5) Different mirror glass, including a heating element
6) Earth connectors from both mirrors back to the car chassis
The existing door mirrors do not contain the wiring for heated mirrors, as shown in this picture of the existing mirror connector within the LH door.
I also bought new heated mirror glass
as it is impossible to retrofit the heating elements to existing mirror glass. This resolved item 5.
Next, I decided to fit the new mirror bodies to the car. This is unfortunately not as easy as would be hoped - to access the mirror fixing bolts the door window glass needs to be removed, followed by the front quarterlight window and window runner. It is then possible to remove the old mirror body and fit the new one. (I'll cover this process in more detail later)
The door wiring looms do contain all the necessary wires for the heated mirrors to work; however the car side of the door electrical connectors do not contain the necessary wiring, therefore this has to be added. (this is from the connectors C-27 & C-28 in the diagram above back to the fuse block F-02) Luckily, it seems that whilst the live (RED) wiring does not exist, the earth (BLACK) wiring from connectors C-27 & C-28 to the ground points G09 & G15 *does* exist.
Although the wiring schematic shows the live (RED) feed to the door mirrors to be located at terminal U in connectors C-27 & C-28, I have been unable to find pin schematics for these connectors, meaning I had to manually trace which pin on connectors C-27 & C-28 should be connected to the live feed from the fuse box by checking for continuity between the mirror heating wires and the un-populated terminals of C-27 & C-28.
Next up was to test the wiring theory by taking a feed from the rear window defogger at connector C-05 and connect it directly to terminal U in connector C-27 (this being the LH door, and so closest to the fuse block and connector C-05). I was then able to test the voltage betweeen the mirror heating wires and the car chassis. It is very helpful that the heated mirrors only operate in conjuction with the heated rear screen, which makes it easy to check for voltage differences when the heating function is turned on or off.
With all my tests completed sucessfully, the next step was to wire the RH door connector (C-28) in the same way as had been done to the LH door connector. As the schematic above shows, both the live (RED) wires connect to a single fuse (fuse 16) in the F-02 fuse block, so whilst one wire has a relatively short run to the LH door, the other wire needs to be routed behind the dash to the other side of the car and the RH door. Naturally the C-27 & C-28 door connectors are not oriented in the same way, so the position of terminal U is not the same between the two connectors.
I'm now at the stage whereby I have working heated mirrors on both doors with a working wiring solution; but I need to find the exact pin specifications for the terminals in C-27 & C-28 so that those wires can be hardwired permanently. I also need a reliable connection to C-05 (I'm currently using a wire-tap, but that's not ideal); and I need to find the electrical connectors used within F-02 so the fuse socket can be added to the spare slot within that fuse box.
Once all wiring pin specifications are known, I can then start work on replicating a retrofit kit which other people will be able to fit to their cars without damaging any existing car components. If anyone knows where I can get technical details of the plugs / connectors used by Mazda, or knows where I can purchase the pins required for the various wires needed for heated mirrors, please let me know!