Window Grid Wires

Shine a strong light through grid from inside vehicle. Check for broken grids which will appear as brown spots.

Fig 1: Heated Rear Window Wiring Diagram Continental, Mark VI, Ford, Mercury Thunder bird, Cougar XR-7
Courtesy of FORD MOTOR CO.
Run engine at idle, turn control switch to "ON", the indicator light should come on.
Using a 12 volt DC voltmeter, contact wide silver strips on back window. Reading should be 10-13 volts. If voltage is lower, window grid ground connection (pigtail on passengers side) is loose.
Contact a known good ground with meter negative lead. Reading should not change.
With negative lead connected to ground, touch each grid line at its mid-point with meter positive lead. A reading of about 6 volts indicates grid line is good. A zero volts reading indicates the grid is broken between the mid-point and the hot side. A 12 volts reading indicates the grid is broken between the mid-point and ground, or that ground connection is loose.