"You can only split the lens from the body if they are euro lights. DOT lights have asolid glass unit, that you could clean through the bulb hole if you had a big enough Q-tip."
Our '91 190E has DOT lights, and they have only a glass lens - the reflector/housing is plastic. However, they are bonded together with what I assume is some sort of RTV silicone. I considered using an exacto knife to separate them, but didn't know if anyone has done this? Since I couldn't afford to have the vehicle inoperable long enough for the new sealant to dry, I did not attempt it. There appear to be alignment interlocks so reassembly would not be difficult, though.
Instead, with the assemblies off the car, I washed them by sloshing non-spotting warm dishwasher solution inside, then rinsing and drying in the SoCal sun (and a hair dryer). I noted while doing this that the sealant wasn't intact - some water dripped out. Maybe this is how the dust gets in, since the bulb holder is sealed.
I have not investigated higher-wattage bulbs for this unit since the reflector is plastic.