Guys,
Just wanted to say thanks, have been reading this (very) long thread and it's been great help in my attempt of installing the aux in cable. Received the cable from Jamie from Sopers (who gave me a good price for the part, thanks!) yesterday and so thought I would spend today trying to fit it.
After several hours, I thought I'd share some of my experience with those yet to attempt, took me a while to remove the vent, it does simply slide out once you have the clips undone, however mine took a little longer as it wasn't giving, then gave it a bit of a pull, turns out that there is some foam at the back of the vent, think with age (mine's on a 04 plate) and heat the foam 'glues' itself to the front of the inner vent.
Once that is off, and the stereo removed, I was looking for the plug that I had to remove and insert the wires from the aux in cable. I didn't manage to find the plug, mine has no CD changer but has both DSP and Bluetooth, so was expecting a plug in the place where I should be plugging the aux cable, but there was none, turns out that I just needed to simply plug in the aux cable. I had already managed to remove the individual wires from the female end of the plug so had to insert them back, luckily I hadn't demolished the plug unit.
I had to then test the cable, my stereo wasn't showing 'Aux in' when I was pressing 'mode' but once the plug was inserted, pressing the 'mode' button did indeed display 'Aux in'. I had to then search for a 3.5mm jack to jack cable, realised that I didn't have one so set out the find one (will spare you the details). Once I had manage to get hold of one, came home tested and it worked fine. I then tried to pop the stereo back in but it wasn't sitting flush with the dash, decided to re-route the wires behind the black plastic 'pillar' in the dash, this did the trick and so tightened the screws. Fitted the vent and routed the remaining of the cable down through to the 'hatch' underneath, where I drilled a hole and popped the female end of the jack through. Pleased with the result, although the sound quality is not as good as through either radio or CD but this is expected using just a 3.5mm stereo plug. I found using the 'Bass Booster' helped and the iPod turned to full volume gives it about the same volume as the other functions. Hope these observations may help anyone else attempting the install, thought it would be good the give something back to the forum!