About a tenner for the bits and about an hours labour for an auto 'leccy Fred.
Nige is bang on
easy job for an electrician or good mechanic..
The problem is the rear screen heater has an unusually low resistance. I measured mine once and it is about half an ohm.. that means..
current = voltage divided by resistance..
12v divided by 0.5 = 24 amps!!! or more.
That's a hell of a lot of current to be feeding all the way from the fusebox back to the heater..
The rear screen heater and the mirrors are on the same circuit, Fusebox tends to catch fire, due to a high resistance connection at the cable to the fusebox termination.. the resistance of mechanical connections tends to increase with age.. oxidization etc.. combine this with pushing thirty Amps and its an electric fire..
The 'Fix' takes a direct feed straight from the battery through a thicker cable via a control relay.
When I first saw the original circuit diagram I couldn't believe my eyes.. crazy because the battery is right next to the rear screen.. designed by a complete idiot.. I can't believe they let it through.. get the fix done asap.
In the meantime do not switch on the rear demister!!!!!!!