Not true grasshopper. The back window can be as dark as you'd like, IF, you have two side view mirrors. This rule applies to every window except the windshield. The driver/passenger side cannot be darker than 50%. If you have only one side view mirror, which some cars in the mid-80's still had, then the 35% law is in effect.

Why don't you people just tint the windows yourself? With the right tools, its not that hard.