Hi Andy, have a look at this thread, there is a photo of mine on there showing the hid and the armoured cable which leads from it to the bulb, also my headlight is upside down in the engine bay but it shows where the hid fits in a well under the headlight, as you can see mine stopped working after rain as it was sitting in water but started working again after it dried out.
As far as I know all other wiring is standard.
I am sure you know but watch out for the 30000 volts the hid uses to start the bulb, once running the xenons only use around 30 watts, they certainly light up the countryside though.
Good luck if you do this conversion, keep us posted.
Just as i posted, you replied
I have actually retrofitted OEM xenons to my previous car, a Vectra 1998, it was actually very easy, as you said the ballast is bolted to the underside of the headlamp, however on the o/s one with the vectra the ballast was mounted remotely in the wing and had a short loom to it which caused a few wiring problems.
However, in theory the only thing different could be the plug shape/pin layout, as you only need 12v and ground to the ballast. There will also be an ignitor somewhere, this may be built into the ballast (in which case the armoured cable comes from the ballast) or it may be seperate (in this case the ignitor has 3 wires from the ballast, and armoured cable to the bulb).
Im hoping if i can find a scrapped car i can look at the plugs and it should be easy. Even better, a page from Haynes showing the wiring, then it really would confirm everything.