Rear defroster not working.

Good afternoon.
The rear defroster in my girlfriends 2000 Olds Alero no longer works. The light on the button comes on, but nothing happens out back.
All of the fuses in the dash and under the hood are okay. The relay was swapped out with an identical one from another circuit... like the blower motor or something. No go.
Testing the voltage on the leads, is where things get weird. In all tests, the car was running and the defroster button was lit:
- Leads disconnected. Attach volt meter. I see about 13 volts.
- Attach leads to heating grid, the voltage drops to 0 with a tiny spark upon connection.
- Disconnect leads from window, and I can watch the voltage rather slowly climb back up to ~13v. (About the speed the counter on a gas pump operates.)
So from what I'm gathering here, it "looks" like some sort of a capacitive circuit is tied into the defroster.
The only thing that has changed with the car, since the last time the defroster was operated (approximately last spring or so), was that the blower motor fan/speed control on the dash was not operating correctly. A shop replaced a resistor up under the dash. I forget the exact name right now. Was it a balast resistor?
Is there more to the defroster circuit than just a button tripping a relay to power the heating element?
Should I dig into the dash, to make sure everything conected securely behind the temp-control section?
Any other idea's where I can stick my hands?
Thanks for your time, -marc
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Make sure the ground wire out of rear glass has continuity to the body.

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David J and Lynne J Shepherd wrote:

Will do... Thanks.
I also hear there is some sort of a defroster / antenna module back there. I'll have to pull apart the molding and check things out.
-marc
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When you pull the plastic panel back on the rear of the car and find the antenna module(rt side), the big black wire going in, then out to the rear glass will be melted ,probably on both sides... No you can either replace it, and find replacement connectors(almost impossible), or try the fix below.. simply splice in before and after the module with a wire as large as the wire there..the current will bypass the module, but the radio can still use th grid as an antenna...I know it's not right, but it works, and saves a bundle.... Bobo

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Bobo wrote:

Awesome! Sounds like this is the problem here...
Thanks, -marc
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when you say you swapped out the relay with another, do you mean the 30 Amp Circuit Breaker in the fuse box ?
========Harryface ======== 1991 Pontiac Bonneville LE ~_~_~270,000 miles_~_~_
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Harry Face wrote:

I'm not...too...sure...
I'm 99% sure what I swapped was a relay. Little 1.5"x1.5" plastic grey box. Some long part numer, maybe 7 or 8 digits. The four terminals in the botom work in a cris-cross patern. One way is open, the other I get a short. I could have sworn there was a schematic on the bottom or on the side or something.
Where would I find the circuit breaker that you mention?
-marc
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30 Amp Circuit Breakers are little silver rectangular, tin boxy looking things plugged in the main fuse box inside the car. They press in just like a regular fuse.
Generally, the Circuit breakers covers Rear Defroster, Power Windows, Sunroof, Convertible Tops, Door Locks and Power Seats.
Seat, Door locks & Defroster are on the same breaker in my car. The second breaker I got covers the Windows and Sunroof ( If I had a sunroof ).
========Harryface ======== 1991 Pontiac Bonneville LE ~_~_~270,000 miles_~_~_
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Harry Face wrote:

Okay, I could have sworn I saw them, but didn't bother with 'em.
I'll check them out just to be sure... but everything else in the car seem to work good, so I'm not too sure.
Thanks, -marc
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Did you check the ground ? Run a jumper from a known good ground, just to be sure.
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