We have had some frost lately and I noticed the rear defogger isn't working on my 99 TB. This weekend I checked for power with a test light and it does have power at the wires on both sides and across all the little wires/elements across the glass. How can it NOT be defrosting? Could it be low voltage? I guess I need to get a multi-meter...
Also, I don't think the heated mirrors are working either, but haven't looked into those yet. I know they have the switch in common, anything else to look for?

 

If you have voltage on both wires, the ground is open, (broken wire, broken terminal, etc). Unplug both wires, ignition in the run position, turn on the defroster. One wire should show battery voltage, the other wire should show less than 5 ohms resistance to ground.

 

Thanks Captain. Diagnosing electrical stuff is definitely a weakness for me!

I got a new test meter and disconnected both wires. I had 11.7 Volts on the drivers side wire (and across the grid when that wire is connected).

Where I'm currently stuck is the display on my meter when trying to check the resistance on the other wire. With both wires still disconnected, I put the red lead on the passenger side wire and the black lead to a good ground; my display doesn't change and reads O.L which appears to indicate over range (pic below). However the meter is supposed to "auto" measure 0 to 20M. I did switch to the continuity function with leads in same position and at least I did not have continuity that way!

I'll continue to tinker with my meter and see what I can figure out.

 

Disconnect both wires from the grid. Connect the black DVOM probe to a good clean ground, no paint, no rust. Ignition in the RUN position, and defroster turned on. DVOM on DC volt range: Red probe to one wire must show battery voltage. The other wire should show zero volts. DVOM on ohms range: Red probe to the zero volt wire must show less than 5 ohms resistance. That's the one I suspect has a problem, in which case, your DVOM will read OL, meaning a bad ground on that wire. Follow it to ground and repair it, or splice on a new piece of wire and attach it to ground.

 

Sounds like a plan. Thank you!

 

Decided to just splice a ground wire in and now have a reading of 3.XX ohms. Am anxious to try it out now!

 

What are you waiting for? Flip the switch and feel the glass after a couple of minutes.

 

Yeah, I tried that day but couldn't feel any heat. got to try it today in real weather, and it works! You are the man!!!

 

Good job. Thanks for posting back.