- Start the engine.
 
- Activate the rear window defogger system.
 
- Connect a test lamp to a good ground.
 
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 goes
out, test the grid line in at least 2 places (1 and 3) to eliminate
the possibility of bridging  the open (2) in the grid line. | 
 
- Once the open (2) is located, repair the grid line.