I'd also recommend getting it done professionally. They tend to carry high quality films, higher quality than what the average shop will have. They also should guarantee their work, which could save you lots of time. Ask around in your area to find a good shop.
Removing old tint is a little bit of a pain on everything but the back window, on the back window it's a serious pain. If you do it yourself make sure you're extremely careful not to tear up the defroster, and as already stated, do NOT use a razor on the back window.
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