Hey, I’ve dealt with a very similar issue on an older Civic I owned, the rear tint had baked under the sun for over a decade and turned into a bubbled mess, just like you described. It’s especially tricky when the defrost lines are involved because one wrong move and they’re gone for good.
From my experience, trying to peel off the film manually often leaves the adhesive behind, which then becomes a nightmare. What worked best for me was using a steamer to soften both the film and the glue. Start at a corner and work slowly with a plastic scraper or even a credit card to lift it without scratching the glass or damaging the defrost lines. It’s tedious, but it beats losing the defrost altogether.
That said, if you're unsure or want to avoid the risk altogether, it might be worth having a professional handle it. I found out the hard way that a botched removal can cost more in repairs. A local shop like
OC Tint Solutions could probably help remove the old tint safely and even reapply high-quality film that lasts much longer. They also offer insight into modern films that resist bubbling and are much easier to maintain.
Best of luck, old cars with baked-in tints are tough, but with the right tools or help, you can definitely restore visibility without losing your defroster!