Hopefully, another member who has actually DONE this project can clarify or correct me.
I have not yet had to replace ballasts ... but .... I found a post on here that says the OEM ballasts are hard-wired into the light assembly and you cannot just unplug the bad ballast and plug in the new and replacements from Infiniti are about $300 each side. His info, NOT mine. That member instructed that you cut the old ballast off at the wires and splice on a new aftermarket ballast. Some of Infiniti parts seem to have been designed to make it as hard as humanly possible to work on. Never would I have thought that you have to remove wheels and dismantle wheel wells just to change a light bulb ?!?!?
If that member is correct, I would suggest you buy BOTH ballasts and change them both out to avoid having to dismantle the car a third time when the other ballasts goes titz-up.
In was able to find a post with pics form a member who installed HID bulbs, fogs, and ballasts. Hopefully this will help. This is actually an install on an M that didn't already have HID headlamps, but if you decide to install from scratch with your new bulbs and cut off the old ballsts, this is a great tutorial.