How do I stop the dog from licking the windows?

My dog gets very excited / anxious when he goes for a ride in the car. During the trip, he obsessively licks the interior windows. It's probably a coping mechanism for his anxiety, but I need a solution that doesn't require psychoysis. Not only is it disgusting, as it leaves a thick film that's difficult to remove, it also hinders my ability to see out the windows! I've tried putting nasty tasting stuff on the windows, but they too are opaque-y and my dog doesn't find anything disgusting, as he routinely eats cat poo. Any ideas?