The only sucessful way I have experienced is replacement.
There is another way. I detail headlights at used car dealers, what I do is drill a small hole on the lower edge of the light and let the moisture out. Using the lights will eventually evaporate the water. To clean the outside of the lens I product I sell called The CrystaLenz Headlight Restoration Kit.
Under the hood by the battery. there will be a black plastic cover with pressure tabs you will need to remove.
The headlight assembly or actually the side light assembly on early SLKs were notorious for collecting moisture. The lens and the glass cover is a sealed unit (not too tightly sealed apparently) and cannot be opened. These were replaced under warranty until you got one that didn't leak and collect moisture. There is no easy way to remove this moisture from the outside.
First remove the wiper and headlight switch knobs. They will pull off with force, then you will under stand how they go back on. Remove cover under dash to view fuses. Take two screw out that hold the fuse panel. The dash cover around the wiper and headlight switch actually pops off when pryed on. Once this cover is off remove the nut around the headlight switch and it will pull out, but you need to get the fuse panel out of the way. The switch it self has a pull out bar that can also be taken out by depressing the button brad on the switch itself.
you have to remove the headlight assembly, there are 2 bolts for each side. To get to the bolts, you'll need to remove the plastic cover that goes across the front just under the hood. After you do that you can undo the bolts and access the bulbs.
There are two ten mm bolts on top in the plastic header panel which must be removed. There are two more under the bumper cover. You must remove the bumper cover to access them.
The radiator fan relay has failed. The relay is under the front bumper cover under the right front headlight. The easyist way to access the relay is to remove right front headlight and drill a 1.5 inch hole to remove. Otherwise the entire front bumper has to be removed.
There are two ten mm bolts on top in the plastic header panel which must be removed. There are two more under the bumper cover. You must remove the bumper cover to access them.
It's under the front right headlight, need to remove headlight assembly and cut hole to get to relay, which is under the plastic frame that holds headlight assembly. It is located under the hood on the left hand side . next to the battery you will see a black or gray box. you should be able to squeeze the cover and pull it off it will tell you on the underside of the cover whioch exact fuse it is.
remove the ram air cover under hood. there are 2 L shaped brackets that hold each headlight in car. lift / pry these out using pliers to gain access to rear of headlight.
you have to remove the two screws on top of the headlight assembly then you have to reach under the headlight between the headlight and the bumper and pull it out.i'd use a piece of cloth to cover the chrome on the bumper this vaw you don't scratch it.once you pull out the assembly then unplug it and change the bulbs you want..
In order to remove the headlight you must remove the whole bumper, that means removing the plastic grill cover, then removing all plastic under the bumper, removing the inner wheel arch, then removing 2 bolts on the bumper, one per side, then pull out, there will be 4 tabs per side, then remove 3 bolts holding the headlight in
If you look under the hood, behind the headlight panel, on the side towards the fender, you should see the motor that runs the headlight. There will probably be a cover on top of it. Remove the cover and you should see a knob that you can turn to move the headlight door. If the door wants to either open or close when you try to move it, just unplug the motor.