Hi there! It's Whitton Car Audio, the guy is very good.
I've just had my JL C3 6.5in tweeters and front speaker upgrade done, £460 total for front inc. labour.
The difference is INSANE, I honestly don't think I'm going to need rear speakers now the front is so crisp.
He also checked the rear audio channel on the stock unit - dead as a dodo. Clearly needs turning on via infotainment, though VW don't seem to think so.
I have a contact at a VW dealership who found a 3rd party company that know how to sort it out in the infotainment system to enable it, I think they quoted about £490+VAT to install and activate the rear speakers?