If you're not careful with the razor blade, you could accidentally cut across one of the defroster elements, rendering it useless.

I've seen the pros strip off the old tint with a professional grade blow dryer (heat gun). The tint is usually easy to remove...it's the adhesive that's left behind that is a mess to get rid of! The blow dryer works best in those situations.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car