OK, just a quick update:
6.5 speakers in front fitted perfectly (mdf mounts needed).
Tweeters replaced oem ones (needed to get rid of some plastic under the cover though).
The rear speaker covers have lots of height, so there was plenty of room for the 7x10s (mdf mounts made again). The only problem was the springy metal things for the boot rub against the magnets of the speakers (they are like dinner plates though).
Now to the interesting bit:
Took the dash to bits and found the oem amp.
The 6 disk changer, which I was led to believe was meant to be Pioneer was in fact a Fujitsu. The slot's in the top and it's a 2003 model car.
Never mind. We broke into the signals from the back of the head unit as per my previous post. Amp remote on wasn't a problem, and we got sound :D
However, we had no control over the volume
Jasper speculated that the head unit sends out a constant signal, and that the volume control affects the gain on the amp. Conclusion: you can't break into the head unit signal and use your own amp.
To get it working, he slapped on a high-to-low converter and did it that way (It was 6.30 by this time, and they were supposed to close at 5.30), but it's hissy and sounds crap quite frankly (not surprised the route the signal takes: low-high-low-high).
Now we've ditched the idea of using the oem head unit and Jasper's made me a facia out of MDF. Going in Weds to get it fitted, along with an Alpine 6893. No idea what it's going to look like, but Jasper seems quite pleased with the results. Will try and get some pictures if anyone's interested.
The moral of this story, is that if you are thinking of running your own amp off the oem head unit in a new IS200, don't.
Taz.