Removing REAL old window tint?

I had an '86 nissan pickup that looked like it had tint as old as the truck. I struggled for little to get the tint off and in the end I just took it to a tint shop and had them replace it with new stuff. They said no problem and didn't charge extra since I was getting new tint anyway. I don't know what they used to get the old stuff off but if you're getting new tint anyway it might be easier to have them do it.