Accy cyclist has it. What you are seeing is plasticiser film, the last problem the motor industry has never solved. It's the plasticiser migrating out of the vinyl trim and settling on interior surfaces. Most motorists don't even notice it and carry on driving with their windows becoming foggier and foggier. The film attracts moisture so makes the windows mist up, disastrous for drivers who don't understand how to use the car's ventilation system. It obscures vison at night and makes the windscreen opaque when driving towards a low sun with a dark background, which is extremely dangerous for cyclists especially.
The secret is to keep on top of it and give the insides of the windows a good polish every month with a clean dry towel. If the film has built up, use water and dishwash liquid and marvel at the filth that appears on the cloth. It's quite tenacious so you'll need to do it a couple of times to get the glass really clean before buffing with a dry towel. You can see it beginning to build up again by wiping the corner of the windscreen with a finger while the sun is shining on the screen. Old cars don't suffer from plasticiser film because they are trimmed with leather and wood. It worries me that I'm inhaling that stuff, which is another good reason for keeping you car well ventilated by always running the fan on its lowest speed.