I had a Ballast fail on one of my headlights and was qouted some silly prices for a replacement so i looked for an aftermarket ballast and come across a lexus ballast replacement on ebay .
Turns out the guy lives not far from me so i collected the ballast with all the required cables to make the ballast fit into the headlight and he advised me on how to install.
So removed the front bumper (quite easy) and removed headlight , bottom cover comes off and has original ballast fitted to it , i removed the original ballast that also has the bulb feed lead connected , its tricky but the bulb feed lead can be manipulated past the AFS assembly.
The original feed for the ballast feeds the new ballast so connected it up and installed the new bulb feed lead past the AFS unit and installed bulb and refitted headlight,
Best £35 ive spent , works a treat and also looked very good quality .
I had a crack in the back of my headlight which had let in water which pools in the bottom of the headlight , which is unfortunately were the original ballast is which is why mine failed but the replacement is a sealed unit so should survive any water ingress.
Link to item below
http://www.ebay.co.uk/itm/Xenon-Hid-Ballast-kit-D2-D4-replacement-Lexus-GS300-GS430-GS450-IS200-LS400-SC-/170979454118?pt=UK_CarsParts_Vehicles_CarParts_SM&fits=Car+Make%3ALexus&hash=item27cf2b6ca6