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2006 SLK350
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I searched and didn't see any posts about this specific topic, maybe it's common knowledge, but I was surprised. We just got off to the start of the rainy season here, and I had my car out on a 40 mile drive last night in the rain. I was almost home when the message display informed me that my left front side marker lamp had failed. When I got into the garage, I checked and yes, the light was definitely not working. I looked at how I was going to get at the back of the light housing to change the bulb, but it was getting on in the evening and I didn't feel like crawling around under the car while it was still dripping, so I let it go until this morning.

After I got the coffee buzz going, I cracked open the hood to see if there was access from above, and it did not look too promising. I took the two bolts out of the headlight assembly that are visible under the hood, but the assembly would not come out, as there is a third bolt underneath that attaches it to the fender. (I found this out after I took a couple of bolts out of the fender liner at the lower leading edge and pulled it apart enough to peek inside). At this point I noticed what appeared to be moisture inside the lens of the marker light. Wonderful!!

I decided after seeing the water in the lens on the driver's side I would check the other side, and yep, there's water in that one too. In fact, it had a definite puddle inside the housing, the front third of the lamp had about a quarter inch of water in it. As this was the first time the car had been in the rain in about 6 months, I thought there must be a problem with the lamp socket or the back of the housing, I never noticed any water in the lamps up to this time.

It's apparent by now that I need to get the lamp housings out and see what is wrong with them. As this was the first time I have taken anything apart on this car other than the plastic shrouds underneath to do the oil changes, I may have done some unnecessary steps, but it worked out well.

I decided it looked pretty easy to get the front bumper cover off, so that was the route I took. I took the front lower shroud between the bumper and the engine cradle off first. The front inner fender liners have two bolts inside the wheel well at the lower front edge, I took these out also. This lets you get at two bolts attaching the bumper cover to the fender. There is a nut accessible through a hole beside each fog light, and there are two bolts beside the headlights under the hood. Finally, there are two bump stops in the center that double as bolts for the bumper cover. Save these for last. Loosen the nuts with a wrench, the screw the whole stop out. It might be a good idea to mark the threads on the stop and a matching mark on the nut to get it back to the same adjustment. Pull the plugs off the back of the fog lights, push out the outside temperature sender on the driver's side, and off comes the bumper cover.

Now that the bumper cover is off, the side marker lamp housings are very accessible. There is a T20 torx bolt at the front edge that holds a retaining device on the back side of the housing. There is also a clip on the back edge as well. Remove the bolt, depress the clip, and remove the retainer, holding the lamp housing in place. Now the housing can be taken out of the bumper. Upon inspection of the housing, it became clear how the water had made it's way inside. The plastic had deformed and cracked right above the light bulb, allowing a way for the water to get in. I guess the heat from the bulb is more that the housing can take, and it is failing. The surprising part is these housings have a mark on the back indicating they are made by Hella, which I always thought made quality products. They missed the mark on these, and I suspect other Mercs using the same or similar housings will have the problem as well.

I sealed the housings back up using the only thing at my disposal at the time, which was hot glue. I guess the better plan would have been to go to the hardware store and get some silicone sealant, but I'm thinking about changing over to clear side markers anyway, so these will probably make it until then.

So, the moral of the story is, if you notice moisture inside your front side markers, get them out and get them fixed, if you leave the water in the housing, the sockets will eventually corrode and the repair will be more expensive, as in take it to the dealer expensive.
 
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