This is the fun part.....

First, snap off the little metal clips, there should be 3 on top and 3 on bottom.

Next you need to heat the goo that seals the assembly so you can slowly prise the two halves apart.    I used a paint stripper heat gun, others have used an oven.   The problem I had with the heat gun method, was trying to get the goo hot enough without making the plastic too soft, then working gently but quickly enough to get it apart before the goo cooled again.  As it was, I noticed later I had bent the plastic a little while trying to prise the halves apart with a screwdriver and the plastic must have been a bit soft at the time.
For that reason, I might be tempted to try the oven method for the second assy

Here you can see the two halves coming apart and the clips I removed.

Here are the two halves, I am currently cleaning the goo out off the joint so I can remove the reflector from the glass in order to polish the inside of the glass.

As you can see, there is a lot of goo to remove, in fact I really doubt that this is the cause of the moisture getting in.    The water would have to go in the slot, around the tongue and back out the slot to enter the housing., that means getting past about 2 centimetres of sealant goo.   I don't think so.

To remove the reflector, remove the screw located just below the parking lamp hole,.

Then carefully pull the reflector away from the front glass. The next photo looks yucky as I didn't use the flash in case of too much glare from the reflector.