Earlier tint products were prone to "lifting" in rear windows with defroster elements (which is what is happening in your case).
To add to that, the adhesive used is far more difficult to remove than the latest tint products.
Tint shops do use a heat gun to remove old tint, but really old tint requires something like kerosene...messy work, and takes awhile! I did it once back in '88, and it was a PITA!
It would be cheaper to just have the tint replaced rather than removed, since most shops will remove the old tint free of charge when installing new.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car