You probably should have asked your installer that 3 years ago. The shop that does mine has been doing my cars for about 15 years. They have a lifetime warranty on their stuff, and the last car that bubbled on the back took about 8 years to start. Having said that, they have a kit that can make connections in the wire elements that have been broken. When you defog your windows, you look and see which ones are not working and they can test them and remake the connections.
Follow the above though and that should be none or minimum. If one or two break, the defogger will still work really well anyway.
Good Luck.
RC