I can't get my car windows clean

Seems like every time I buy a pre-owned vehicle from a dealer, I can never get the windows clean. I don't know what the heck they put on the windows but I'm thinking it's some kind of wax. I bought my car almost two years ago and I swear, the windows are still mucked up. It doesn't matter how many times I clean them using Windex or vinegar and water, my windows never get completely clean. They're always smeared and when the sun hits them just the right way, they are a mess. 1) What product do they use on car windows at dealerships? 2) How do I get this crap off ? I'm listening to any good advice to solve my problem.

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