On older Mercedes-Benz models, the ballast and ignitor were mounted under the headlight. Typically this required that you either remove the headlight or loosen the front bumper to access the ballast. On newer Mercedes models, the ignitor is mounted right on the back of the HID bulb.
Open the hood and access the back of the headlight.
Remove the cap by twisting it counterclockwise.
Unplug the wire from the ignitor.
Twist the ignitor counterclockwise to remove it.
Install the new ignitor. Line up all four locking tabs then twist it clockwise to lock in the headlight.
How to troubleshoot Mercedes ignitor
Common signs of a bad Mercedes ignitor is that one of the headlights are out. When the ignitor starts to fail, it will work randomly. If the HID bulb is burned it will never come back on or work randomly once it fails.
First, check the low beam fuse. Mercedes uses a fuse for each of the headlights. Therefore you have a left and right headlight fuses.
If the fuses check out and are not burned, you need to check the ignitor.
Swap the ignitor with the other side.
If the headlight where you moved the ignitor does not work, the problem is the ignitor.
If the headlight where you moved the ignitor does work, the problem is NOT the ignitor.