The parts came today and since it is a dangerous pain driving without an offside mirror I decided the job qualified for that most traditional of classic car repair methods - doing it in the dark and rain after work.
I discovered quickly that the glass was actually missing altogether. It seems it’s another case of Volvo using a glue that doesn’t last the distance. This also explains why the mirror had started wobbling a lot. It wasn’t the motor broken but instead the glass parting company with its backing. This thread on a US Volvo site suggests it’s quite common there. The OP, like me, also seems originally to have thought combustion was involved. I think this is because the departing mirror leaves remnants of its printing in the black glue and looks burnt.
https://forums.swedespeed.com/showth...f-mirror-glass
My spare parts seemed good quality. See the first two attached pictures.
https://www.ebay.co.uk/itm/Volvo-199...t/282260347637
I think this mirror glass compares favourably to others on eBay in that it looks more nearly OEM.
https://www.ebay.co.uk/itm/Volvo-S70...n/262745787992
The third picture is just there to show how on a cold night, with minimal care, it is possible to remove the new style mirror with all the clips intact in under a minute. This is only doable if the mirror is already a write-off!
I probably didn’t need to replace the motor in the end because the upheaval of removing the mirror backing caused it to start working again. However, I thought it was worth doing anyway. The new motor has a strange anomaly whereby up and down work correctly but left and right are reversed. If it had not been cold and wet I might have investigated this further but under the circumstances I decided to accept this as the new normal. It seems strange because the three pin connector is keyed and will only go in one way and it is not possible to reverse the motor left/right without fitting it upside down which would also be near impossible given the position of the three screws. Besides, up and down work correctly. I guess I could swap pins at the switch to correct this if I really wanted to.