Hope this might help some other poor puzzled owner at some point. I posted a question a while ago when I had a problem with the heater fan suddenly starting at high speed even when the switch was off. I could hear arcing and assumed at first that damp was getting at the back of the switch. I removed the radio to feel the back but it was quite dry so I took out the pollen filter so I could feel the matrix for any damp. Still all ok. After much head scratching and almost at the point of taking the car to a main dealer, I took the radio out again to try and see any visible wiring as the arcing sound seemed to come from there. At the back of the cavity a bunch of wires marked heater was tucked behind a steel tab with a hole which is a guide for a lug on the back of the radio. This tab has a sharp back edge, and over the 5 years since the wires were put there during manufacture, one had finally rubbed through the plastic and was shorting on the tab! Job done, but what would a main dealer have charged me?!