Just to let you all know that after some digging around it would appear that these units (Bi-Xenons) are sealed but can be persuaded open with the clever use of a hairdryer!
I will see if the misting is too bad for me to endure before taking this drastic action.
Replacement units are particularly costly!
Mac.
Most of the research I have done on the condenstion problem points to bad design. Your headlamps are not faulty i.e. they are not leaking around the seals. Allowing air to circulate stops the problem. I removed the fog light bulbs in mine with their covers as an experiment, and the condensation stopped. Not an ideal solution but air circulation solves the problem as the headlamps need to breath, and larger vent holes in the headlamps would seem to be the answer. These can be modified if you are good at DIY, and brave enough too!
This is a problem on lots of cars, not just Mercs.
When the vehicle is parked and the lamp is turned off, the ambient temperature inside of the lamp housing gradually cools, and the air left in the housing contracts, thereby lowering the pressure inside the housing and drawing fresh external air into the housing through the vent (s).
When conditions outside the lamp housing include a high level of humidity, the replacement air drawn into the housing is humid air, which may contain tiny suspended water droplets and/or evaporated water in the gas phase.
As the lamp housing continues to cool, moisture, from humid air which has been newly drawn into the lamp housing, may precipitate and condense out of the air to form liquid water, and such water may be deposited on the internal surfaces of the housing.
Once a high level of moisture becomes established inside of a lamp housing, it may persist and be difficult to get rid of, because the vent hole or holes are relatively small, and turning the lamp back on may cause re-evaporation of liquid condensate within the housing. A cycle of condensation and evaporation inside the lamp housing may follow, without significantly reducing the humidity level therein. Repetitive cycles of evaporation and condensation, under the above-described conditions, may actually exacerbate the problem and promote deterioration of electrical components of the lamp.
Moisture buildup in headlamp assemblies results in customer dissatisfaction and an expensive replacement of the entire lamp assembly. Such condensation can occur immediately following production or can show up much later in the useful life of the vehicle.
There is a trend in the automotive industry to mount the vehicle headlamp in an enclosed plastic headlamp assembly. A halogen or tungsten lamp is mounted within a plastic assembly, which contains the reflector, lens, positioning mechanism, and lamp fixture. In addition, there has been a trend toward the use of clear plastic lenses, without texture or embossing, in such headlamp assemblies. Moisture buildup becomes much more obvious with this type of lens.
Check this out too...
http://www.wipo.int/pctdb/en/wo.jsp?WO=2001/055636&IA=WO2001/055636&DISPLAY=DESC
