I did an HID retro using the shielded bulb holder assembly from a 2003 Volvo V70R wagon. Basically I took apart the Volvo headlights to get the shielded holder, bulb and ballast. The 4th gen headlights needed to be taken apart, the existing shield taken out and the hole for the bulb had to be made larger to accept the Volvo assembly.
The results were a descent set of HID's with no glare and good beam down on the road. A few guys on here have asked for pics so here goes.
First, the shield and D2R bulbs.  Next you install them into the 4th gen housing. (use ./readarticle.php?article_id=75 to open headlight) 
The results....  
  -njmaxseltd |
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