To locate inoperative grid lines, turn the ignition key to the ON position and turn the rear window defogger switch to the ON position. Attach the black lead of the multimeter, set to the 20 volt scale, to ground. Lightly touch the red lead of each grid line.
The multimeter should display battery voltage at the positive side of each grid line. The voltmeter should gradually display less voltage as the red lead of the multimeter is moved toward the ground side of the vehicle.
If the reading shows battery voltage as the red lead is moved across the grid lines and then makes a sudden jump down to zero volts, ground, that grid line has an open in it.