Did the same about a year or so ago when I installed some '13 takeoff HIDs. Passenger side would fog up like a **** with the slightest bit of humidity & rain.
Got to researching it and got the idea off the Hyundai Genesis forums of all places. They have a similar problem. Took the assembly out and resealed it all with silicone and then tossed a desiccant pack in the same place. Over a year later and the lens has never fogged once in any conditions.
Ford will tell you a little condensation is normal. I will tell you that is absolute BS and it's only normal because their quality control has deemed it OK to accept light assemblies from their vendor with **** poor seals on a certain percentage of them. It's luck of the draw. How can you have certain assemblies that never fog in their lifetime and others that do. If you can get it right most of the time you can get it right all of the time with some effort. Ford tells their customers it is normal and they accept it.
Edit: saw the OP was talking about retrofits so I guess all bets are off as those are cracked open to be modded. My beef with Ford OEM lights still stands.