Defogger Grid Lines Diagnosis

WARNING: This page is about a different car, the 2003 Oldsmobile Bravada, 2003 GMC Envoy XL, 2003 GMC Envoy, and 2003 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.

Start the engine.
Activate the rear window defogger system.
Connect a test lamp to a good ground.

Fig 1: Identifying Test Lamp Probe From Zone 5 To Zone 1 Along Each Grid Line
Courtesy of GENERAL MOTORS CORP.

IMPORTANT: The test lamp brilliance will decrease proportionately to the increased resistance in the grid line as the probe is moved from the battery positive bus wire to the ground bus wire. The test lamp brilliance may vary from one window to another.
Move the test lamp probe from zone 5 to zone 1 along each grid line.
- If the test lamp shows full brilliance at both ends of the grid lines. Inspect for an open or poor connection in the ground circuit of the rear window defogger grid. Refer to TESTING WIRING SYSTEMS FOR INTERMITTENT & POOR CONNECTIONS and CONNECTOR REPAIRS in Wiring Systems.
  
  Fig 2: Identifying Test Grid Line In At Least 2 Places
  Courtesy of GENERAL MOTORS CORP.
- If the test lamp goes out, test the grid line in at least 2 places (1,3) to eliminate the possibility of bridging the open (2) in the grid line.
Once the open (2) is located, repair the grid line. Refer to GRID LINE REPAIR .