Causes
So just what is that cloudy gunk on the inside of your windshield? Well, many things can be the cause including condensation, dust, and tobacco smoke. The most common cause though is the outgassing of plastics, foams, and vinyls used in producing vehicle interiors. This outgassing is accelerated when the interior is hot and is most often seen on new cars. As vehicles age this outgassing fades away and is usually not long term.
Removal
The best method to remove the film is to mix a solution of one part white vinegar and ten parts water. Apply this to the inside of the windshield and simply wipe it clean. Be very careful to not get any on the interior fabric and plastics to avoid possible staining.
Prevention
To help prevent film build up, keep the vehicle in the shade and leave a window slightly open during hot weather. A windshield sunshade can also help in prevention.