When I bought my car the tint that was in it was cheap "China" tint and had turned the usual nice shade of purple.
I removed all my tint myself, its not hard to remove the tint...its hard to clean up the mess left behind.
All I did on all of my windows was grab a corner and pull. Once it was off, I used a nice sharp razor blade and some Imperial Cleaner and that made clean up easy. I did not use the razor blade on the rear window, I did use a really fine scotch bright pad though, and with the cleaner...it was easy as pie.
Making a tint shop "replace" or "fix" your rear window if they f-it up is slim to none. Your better off doing it yourself. Plus, it will make the price of the new tint cheaper if all the old tint is removed and the windows cleaned.
I had my car tinted with commercial grade metallic film, that is 20% and meters at 17%, the entire rear glass was done in one solid piece of film, so I have no seams or gaps, I paid $100.00 with a life time guarantee.
~F~