With the greatest of respect, this is not a job for the faint-hearted!

The heated front screen draws more current than anything else on the car
apart from the starter motor. I believe it is around 40A? When you switch
it on, the lights dim, the blower fan slows and you can hear the alternator
whine in protest!

For that reason the screen elements are in two separate circuits. They are
wired so they can only come on when an "engine running" signal is sent by
the ECU. At the same time, the idle value in the ECU is increased to keep
the alternator output high. The software is possibly present in all cars
whether fitted with a HFS or not, but you would need to confirm this before
you start. I don't think any of the wiring or relays are fitted either.

Others have asked this question here in the past and the general opinion has
been that it is simply too much trouble to retrofit.

If you decide to go ahead I would urge you to get it done professionally
unless you feel you have the skills needed. Let us know how you get on.

Chris.