My ABS light is on also. I was told it's because I have bigger tires on the back. The back tires are 255/50ZR17 and the fronts are 235/45ZR17. So the tires are spinning at different speeds and the ABS turns itself off.
Not sure exactly what the abs thing is with our car but there are two theories. First is the location of the ballast must be emitting some kind of EMF that the bothers the abs sensors. I have heard that relocating can help this sometimes.
The other is that I know when the ballast initially fires, the current consumption will spike around 12-13 amps, and then settle down to 6-8 amps and the fluctuation in amperage somehow makes the computer think that something is wrong with the abs. I have heard that installing a relay can fix this.
I have never had an abs light come on in mine and I have had hids for 5-6 yrs now but I did do a relay harness for both sets(head lights and fogs). I had also heard that putting them behind the battery was a safe location for them and that is where I put them. Not sure what makes the difference but neither one was difficult to do.