Hi mate,
Well at the moment, or with most if not all aftermarket headunits, not only do you get pre-outs, but you will also get a 'remote amp' wire, which you connect your wire from the amp which tells it when your radio is on etc!
So if you sort the speaker adaptors out to get around the lack of RCA connectors on the VW headunits, you still need to find a home for the remote wire from the amp.
So, connect it to a switched live (ie comes on when you turn the car on and off when you turn the car off) to get around it.
There is a switched live wire, behind the climate control panel (where you set the fan speed etc) when you have the radio out, it's easy enough to just undo the screws holding that in place, tap into the wire on pin 20 and your jammin!