Grid Test

Shine a strong light through grid from inside vehicle. Inspect grid from outside. Broken grid lines will appear as brown spots. Run engine at idle. Set control switch to ON position. Indicator light should come on.
Connect negative lead of a voltmeter to ground. Touch positive lead to wide red-brown strips on rear window. Meter should indicate 10-13 volts. If voltage is lower, window grid ground wire (pigtail) connection is loose. Contact a known good ground. Reading should not change.
With voltmeter negative lead grounded, touch each grid line at its midpoint using voltmeter positive probe. A reading of about 6 volts indicates grid line is good. A zero reading indicates grid line is broken between midpoint and battery side of grid. A 12-volt reading indicates grid line is broken between midpoint and ground.