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jg - you are a genius mate ! I woke up this morning, thinking to myself... hmmmmm maybe the mirror is electrically heated and I simply didn't know about it. I spoke to a couple of my work colleagues who both said straight away, "oh, the wires are for the mirror heater/demister".... and then the penny dropped - I was thinking, that jg is a very clever person, so thanks again !
As soon as we took the old mirror glass off, we could see a little element. The wires simply pulled off their holders and we were able to replace the mirror with the new item (we wrapped up the connectors in waterproof tape and tucked them out of the way). Now, if I find I really do need the heater element, I'll try to buy the correct replacement glass ! I took a note of the serial number on the back just in case.
Interestingly enough, I also found out where all those spiders had come from .... lol