Service history is key. Make sure there's proof of gearbox quattro system and engine services. Check for any smoke out the exhaust. Black could indicate a blocked dpf.
It's dpf and egr which are the expensive bills. Make sure gearbox changes are smooth regardless of manual or auto as clutches/dmf are expensive bills.
I believe they like to eat suspension arms so look out for any knocking noises.
Make sure all electrical items work as head units or AC compressors can be expensive bills.
As cliff has suggested I'd recommend a diagnostic check to see if any fault codes flag up. Otherwise we're guessing. But could be issue with maf sensor, lambda sensor etc.
This should go back to main dealer for diagnostic check to see if any fault codes flag up.
In the handbook it should tell.yiu how to do a system reset on it. Might be worth trying that also.
Quite a few on eBay.
https://www.ebay.co.uk/itm/355110793527?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=Qn30XPa8QES&sssrc=4429486&ssuid=IE07kO4STZy&var=&widget_ver=artemis&media=COPY
VW Tiguan or Skoda models are the same.
I appreciate this is for an A6 c8 but I think the process is the same:
https://www.audiworld.com/forums/a6-c8-platform-discussion-229/disabling-auto-dimming-mirror-feature-3007601/
Welcome to the forum you'll find the members on here are a friendly and helpful bunch 🙂
Glad you got the problem sorted.
I'd recommend you try eBay for the mirror.