Old method is to turn it on, then take a small piece of copper strip and use it as a trace to go along each strip to find the breaks. You attach ther strip tp the positive lead of the multimeter and the negative to a good chassis earth, then slowly wipe along each strip. A good strip will start at 12V and gradually change to 0 as you move along. A bad one will be 12V until you hit the break, then drop to 0V. Have a dry wipe marker to mark each spot, then get the repair kit and do each break.
A long job, but the only alternative is a new window glass, or strip it off and a aftermarket demister strip panel.
I only wish the previous owner of my car had done that, there are 5 breaks, and they occurred before the inner solar film was installed. Remove that and I need a new rear window.