What a fantastic post and as I read every one of your posts , I thought, yup , thats the same as my problem so I continued through to the end and was really hoping you would sort the problem, which of course you have done.
Sadly , my car does not have a sun roof, so my woes are not going to be answered by your solution. Nevertheless some of the other suggestions may well be worth following up, but my deep down fear is that is a leaking heater matrix.
This afternoon , with all carpets peeled back, I sat and ran the engine for about 30 minutes but there was no sign of moisture at the front bulkhead , inside the car obviously.
So next avenue of investigation will be the areas behind the wheels and behind the liners.......
Congratulations on your fortitude and sorting the problem , as well as writing a very useful post.