Overview:
The difference between the early style xenon and later style is that the early one uses a wing mounted remote ballast and the later type has the ballast mounted to the rear of the headlight unit directly.
Early type xenon:
In the above picture plug 1 is for the remote mounted ballast pack (pictured below) connector and plug two is for the headlight loom 12 pin plug
Late Type Xenon with ballast mounted:
The later type ballasts are more readily available than the early ones when it comes to replacement but do not fit the early lights, it is however possible to upgrade the early model lights to the late model ones.
Parts:
You will need three items to perform this upgrade:
- 1. The rear cover off the late model lights, this can be either of a damaged unit (scrap yard or eBay) or you can buy them from VW for about £40, the part numbers are in the diagrams below - item 6
- 2. An ignitor from a late model xenon - these are not available for the Golf directly from VW but are for the Bora (1J0 941 641 B) and are exactly the same
for £85 
a much better source is Ebay where they can be had for £25ish (BMW and some Vauxhalls use the same ignitors)
- 3. A late model ballast pack, these can be had from VW for silly money or from Ebay \ Scrap yard
The cheapest route to get everything is to buy a late model xenon that is damaged
The upgrade:
This is the early style xenon:
The ignitor (1) has a 2 pin multi plug (2) fitted to it with a blue and red wire in it. The bulb holder (4) is attached to the ignitor by two rubber coated wires. The multi plug (3) is where the remote wing mounted ballast connects to.
Open the rear cover and disconnect all the wires, there is one to the levelling motor inside the light that can be tricky to get at un screw the ignitor from the rear cover and remove the rear cover.
You will be left with the ignitor to remove, the bulb holder will not pass the case so you will need to cut the wires (mark then first so you know which is which), cut them at the ignitor end if you are replacing with an ignitor of a different car as they can have shorter leads so you will need all spare cable later.
With the ignitor removed you will be left with two cables that connected to the external ballast, these can be removed or secured out of the way internally.
This is the new rear cover:
Note the ballast connector circled in green and the mounting holes for the ballast in yellow
The three pin ignitor plug with the red, green and black wires is circled in red.
Cut the bulb holder off the new ignitor at the bulb connector end - again mark the wires, and put some heat shrink tubing down over the wires attached to the ignitor.
Thread the new wiring through the case to behind the xenon holder and solder the bulb connector from the old xenon (longer wires) to the new ignitor ensuring the wires are connected correctly then push up the heat shrink tubing to insulate the joint and heat it with a hot air gun. I used some insulation tape as well jut to be sure
Secure the ignitor to the rear cover and connect the wires \ install the bulbs + fit the ballast to the rear of the light.
Job done your light is now late model one
While you are doing this you may want to do the rheostat mod as well to the lights
HTH
Neil