Here's an effective way to track down an electrical short in main parts of the electrical system: Connect a 5-amp circuit breaker across the fuse block terminals of the affected circuit. With the circuit activated, probe the various parts of the circuit and connectors with your test light. If the light blinks on and off, continue to the next portion of the circuit. When you get to a point where the test light doesn't come on, the short will be between that point and the last point where the light flashed.
To check the power supply, connect the clip end of a test light to one end of the grid buss bar and touch the other buss bar with the probe end. (You can also do this with your multimeter). If the light comes on, power and ground are OK. If the light doesn't come on, connect the ground clip to another ground. If the light now comes on, there's an open ground circuit. If the light still doesn't come on, check the fuse and connections.
To check the defroster's heating ability, connect the clip end of the test light to a good ground. Now, touch each horizontal bar at the supply end with the probe. The test light should glow brightly. Continue moving across the bar to the ground end and touch the bar gain. If the bar is working correctly, the light should dim as you move towards the ground end. If the test light brightness stays the same as you move across the bar, you've found one with an open circuit.
As with most things, the best approach is usually the simplest. If you remember this, you can keep your productivity at its highest, with the lowest amount of lost diagnostic time and comebacks.