I was mistaken when I thought it was the bumper harness. Assuming your car is a B207 LHD it's front harness #12804574. It's $272 new from esaabparts. I'm pretty sure it includes the wiring for the front level sensor. I can't find rear axle, passenger compartment, roof, or parcel shelf harnesses that are distinguished by XENON, so perhaps the wiring for the rear sensor is already in the '05. The front sensor, #93187465 is $121 new, and the rear, #93186514 is $99 new from Schmelz Countryside VW. Used would be cheaper of course.
I won't hack my car so when I do something like this I go factory where possible and you have a great start by getting OEM headlights for a great price. This is the kind of mod I love to do, and I plan on doing my '05 convertible. If you do this keep us posted.