The rear defroster works on a resistive element principle; current is passed through the grid, and since resistors work by transforming current into heat, the grid heats up, defrosting the window. If there is a short anywhere in that system, generally a fuse will blow, but the most practical result is that all of the electricity will go through that short, and not through the resistive grid. The grid is also a parallel setup; a break in any of the lines will not only cause that line to not work but cause the overall resistance to go up - too many broken lines and the grid will have so much resistance that it just won't heat up enough.
So the first thing you need to do is comb through the grid very thoroughly for the smallest of nicks, cuts, breaks, etc. that might cause the grid to malfunction. If you do find cuts or breaks, there is an easy way to repair them without replacing the glass. You go to Radio Shack and buy a pen that contains conductive silver-based fluid. Just ask the counter guy for the circuit writer pen; it will be advertised for drawing circuit paths on cards (something it happens to also excel at). You can simply draw on the window between the ends of the break in the line (on the inside, obviously) and restore continuity. The part that you drew will not defrost as well since the pen is designed to have as little resistance as possible, but having 2mm of non-defrosted area is far better than having an entire line - or no - defrosted area. I have done this to great effect in my own car with two breaks caused by an errant dremel cut-off wheel (don't ask).
