I did this on my old 5.1 Velle. The access port I think is for the VW oem MDI device which sort of did USB in a bad way. I ran an aux and a USB in there, but in reality just a USB would have done fine. Beware that if you don’t use the MDI slot ( at the back of the glovebox) and go in via the side where I think there are perforations it can be a bit tricky getting the box back into the dash. Oh and watch out for VW’s beautifully stamped steel bits if you’re jiggling things with your hands up from the footwell. Blood was definitely lost!
DO NOT turn the vehicle on with the airbag warning light or airbag disable switch disconnected! It won’t go BANG! but it will throw an error that has to be reset with a code reader. The passenger airbag activation wires are taped to a bit of steel just below the box. They’re bright yellow and bloomin obvious. Easy to snag on a watch.