3 door rear audio options

[th3_f15t]

Hey, double edged question here...

Upgrading all the speakers in my Mk5 GTI, and need to know if I'll need adaptors to mount aftermarket speakers in the rears? I'm confident to say the rears are a 2-way 165mm system? Correct me if I'm wrong...

Secondly, is it worth installing an amp and wiring the rears to that as opposed to using the head unit? Using a 3 channel amp, allows for addition of a sub at a later date (or bridge a second part of components).

[th3_f15t]

I've been looking at this and I think these are needed for fitting rear speakers;

http://www.caraudiodirect.co.uk/install-adaptors/volkswagen/golf/ct25vw01-speaker-fitting-adapter-for-vw-passat-golf-iv-front-and

Anyone know anything better?