After reading posts here and much research, I thought I'd share how I got round compatibility issues with standard head units, wheel controls, speakers and glovebox cd changers in my 2005 Mk4 1.8 convertible.
  • Fitted Infinity speakers in the front doors (woofers and tweeters with crossovers).
  • Then wired in a Kicker amp in the boot, using rca cables through a conversion out of the speaker outs on the h/u.
Everything works as normal and it sounds bloody good. Only drawback - without ignition switched on (and therefore the amp being off) sound only comes out of the rear speakers (which are crap anyway!)
Easy to do, easy to pay for (amp cost £90 and the speakers £65 off ebay) and everything is good.
OK may not be the best ICE install around, but if you want a normal but quality sound system using the standard Vauxhall H/U, cd changer and wheel controls, this does the trick.
happy to advise anyone who wants to follow suit.