I don't know what they used for the old tint, but when I got the tint done on my daily driver, the guy said not to use ammonia-based window cleaner on it, as it is what pros use to remove old tint. I've not used it, so I can't say for sure, but if he's right, just try some ammonia, might just take it right off.
Like Alan said, though: Do it in the fresh air, don't want any casualties :P
[EDIT] Sorry to bring up an old post, forgot to read the date
