Grid Wire Test

WARNING: This page is about a different car, the 2007 Mercury Mariner and 2007 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.

Using a bright lamp inside the vehicle, inspect the wire grid from the outside. A broken grid wire will appear as a brown spot.
Run the engine at idle. Set the rear window defrost switch to ON. The indicator light should come on.
Working inside the vehicle with a voltmeter, contact the broad, red-brown stripes of the back glass window positive lead to battery side and negative lead to ground side. The meter should read 10-13 volts.
Contact a good ground point with the negative lead of the meter. The voltage reading should not change. A voltage reading of less than 10 volts indicates a loose ground connection.
With the negative lead of the meter grounded, touch each grid line of the rear window defrost glass at its midpoint with the positive lead. A reading of approximately 6 volts indicates that the line is good. A reading of 0 volts indicates that the line is broken between the midpoint and the B+ side of the grid line. A reading of 12 volts indicates that the circuit is broken between the midpoint of the grid line and ground.