MKIII? Hmm, you will lose some functionality there compared to the MKIV...
Behind the left seat (looking from the boot) is a connector. The harnass clicks on top of that (piggy back) and goes from there to the DVD computer (put it in the boot or so or on the floor somewhere). Plug the GPS antenna in the DVD unit, plug the screen in at the other end of the cable, plug the other connector in the radio (pin 5 and 11 I think) and put in the new CID. You have to connect the new CID instead of your old radio though.
You can test the whole system without removing a panel, except for the radio ...