New Headlight assembly - have questions

The ballast is external to the housing. It's a little gray piece clipped in by the radiator with a wire running from it to the housing. you wont have to transfer it over, just unplug the old and plug in the new. as far as having a bulb out, start by checking the fuses. there are two pairs for the low beam/hid headlights: one labeled low beam headlights, the other labeled HID. Both of these have to be good for the headlights to work. If all the fuses are good, check the bulb next. You'll have to take the bulbs out of the existing housings to transfer them over anyway. I try to avoid doing this as much as I can because it is a pain to do and I always feel like I am going to break the bulb. (Depending on how much you are willing to spend, it might be worth picking up a new set of bulbs anyway so that you wont have to ever change them again for the next 20 years or so.) The HID bulb is coming out of a small metal box that plugs in to one of the wires inside the housing. there is a little metal spring clip holding this assembly into the socket. Release the clip and very carefully fenagle the bulb assembly from the socket. Sometimes it is easier to unplug it first, sometimes it is easier to unplug it after, but it is never easy. Once you have the new housing plugged in, move the "good" bulb to the "bad" side. If it turns on, then you most likely have a bad bulb. you can confirm this by moving the bad bulb to the good side, and it should not come on. If the good bulb does not come on when on the bad side and the bad bulb does turn on on the good side, then you most likely have a bad ballast.