Fairly easy to do with the correct adapters. I installed an Alpine UTE-204DAB using these 2 adapters and it was plug and play, however, the quadlock adapter is fairly pricey but was well worth the money in my opinion. If you were to go down the route of cutting wires it's always better to buy an adapter harness and cut that up rather than the factory wiring.
ISO to DIN:
https://www.amazon.co.uk/gp/product/B00 ... UTF8&psc=1
Quadlock adapter:
https://www.amazon.co.uk/gp/product/B01 ... UTF8&psc=1
The switched power I believe is done via CANbus on the 9n3 so if you were to not use an adapter you would have to find a source of switched power and splice into it. I think the closest one would be the cigarette lighter.
I followed this guide and it may be of help if you choose to not use the quadlock adapter.
https://forums.mightycarmods.com/forum/ ... olo-gti-9n
Radio keys:
https://www.amazon.co.uk/gp/product/B07 ... UTF8&psc=1
Also as it'll be a single-DIN you'll need a single DIN pocket, you can either get a universal one or an OE one from your VW dealer. However, the OE pocket was about £40.
OE pocket part numbers:
6Q0 857 058 C 1QB - Single DIN Pocket
6Q0 858 167 A9 B9 - Single DIN Pocket insert
Universal pocket:
https://www.amazon.co.uk/gp/product/B00 ... UTF8&psc=1
If you want to fit that universal pocket and want to be able to easily remove it don't put the locking tabs on as the only way I found to get it back out was to pry it out.
Hope that helps
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