No you dont have to replace your whole window, thats how they steal your money! >

>
But seriously, I have the same problem. My old window tint was bubbly but there was no way to take it off without scratching the defroster lines. So i pretty much scratched off the entire defroster.
If you take a razorblade and scratch off the entire defroster, you can then put on a replacement defroster and tap it into the defroster tabs (metal ends that touch the lines and create the electrical flow of heat).
You can purchase the kit from
http://www.frostfighter.com/ff_faq.htm and either install it yourself or have one of their "certified" palces near you do it.