[clarkey1984]

There are 2 systems fitted in these cars that include the sub, one is the more basic setup where just the sub is powered by the inbuilt amp, and all the other speakers are powered by the connectors in the grey plug, and then there's the premium system, where the door speakers, rear speakers and the sub are amp powered, so only the dash tweeters are powered by the aforementioned connector, I discovered the 2nd system when fitting my aftermarket unit into my old SE while the T4 was busy having a broken engine, all I had was tweeters.

however, so far as I understand it, in both cases to get a signal to the amp there is a din connector, as that's what drives the amp, for that to work it will indeed need to be powered, the 12v blue remote wire on aftermarket units being the perfect thing.

you'll need to have at least one set of sub / amp RCA pre outs on your new stereo for the signal though, ideally 2, but if it only has one set you can always use splitters, the main part you'll need is the din to RCA powered adapter, which is autoleads pc9-414, that's what I used in my old 52 plate SE which had the later system, the sub was broken in my 02 T4 that has the earlier system so i never bothered with it, i hope this helps.