I recently had my passenger side low-beam go out, so I decided to replace both D2S bulbs since they're pretty old. Everything went together normally on the passenger side, but I discovered the driver side bulb was totally loose in the headlight assembly, i.e. the bulb, igniter, and retaining ring weren't locked into place. When I try to lock it back into place, I can't get the retaining ring to turn clockwise and lock into the headlight assembly once the tabs have gone through the slots - the only thing that turns is the igniter relative to the ring, which doesn't accomplish much regardless of whether I lock the two together before or after putting the retaining ring into the headlight assembly. There doesn't appear to be anything broken on the retaining ring. I'd take the headlight assembly out to get a better look, but I don't think I could get the front bumper off in my garage.

Has anyone experienced anything like this and found a solution? What could cause this problem?

Yes, I know exactly what you're going through. I had the same exact problem. I even bought the replacement plastic ring in advance assuming that the older ones in the car would be cracked or broken and even the new ones would not slip in properly. I have to kind of push them in place but then the headlights are not aligned and on dark nights I end up using my brights instead. I don't understand how Mercedes designed these things to be so complicated. The other scary part to this is the three wires to the d2s igniter. Extremely thin and the insulation disintegrates the minute you touch the headlight. I recommend before changing any headlight you need to buy liquid tape clear , only the clear not the black, and using a small long thin brush carefully apply the liquid tape to those three wires to secure the insulation around the them Then let it dry for a couple of hours at least so that your wires are safe

I still haven't gone back to fix the headlights which I hope to replace with LED in the future

Here are some pictures. This whole thing originally should have taken 5 minutes to replace a bulb. Turned into a 3-hour fiasco. The horror the horror

Yes, I know exactly what you're going through. I had the same exact problem. I even bought the replacement plastic ring in advance assuming that the older ones in the car would be cracked or broken and even the new ones would not slip in properly. I have to kind of push them in place but then the headlights are not aligned and on dark nights I end up using my brights instead. I don't understand how Mercedes designed these things to be so complicated. The other scary part to this is the three wires to the d2s igniter. Extremely thin and the insulation disintegrates the minute you touch the headlight. I recommend before changing any headlight you need to buy liquid tape clear , only the clear not the black, and using a small long thin brush carefully apply the liquid tape to those three wires to secure the insulation around the them Then let it dry for a couple of hours at least so that your wires are safe

I still haven't gone back to fix the headlights which I hope to replace with LED in the future

Here are some pictures. This whole thing originally should have taken 5 minutes to replace a bulb. Turned into a 3-hour fiasco. The horror the horror