kklou,
I know the film you are talking about. I just keep a cotton cloth inside the car with me and when it becomes apparent I wipe affected areas with said dry cloth. It takes so little time to cure (once a week quick-rub) that I've not looked deeply into preventing it altogether. The film formation does not seem to care whether the car is clean, dirty, or anywhere in between. Have heard it is elements of the vinyl dash and trim "off-gasing" and hence leaving the stubborn film. BTW, I'd avoid RainX interior window treatment. I gave the stuff a try and although it did seem to help with interior fogging, it left a film that was a real %(*#%(& to get off.

Hope this helps...Lee