I just recently repaired my 2000 Park Ave. With the same problem. I purchased a GM OEM C-Platform Service Manual that covers most Park Ave's of the '97-'05 vintage. The service manual was a little confusing but I'll guide you through the steps.
First question is: Have you noticed a loss of AM/FM radio signal strength or a lot more noise just recently? The reason I ask will be explained as we step through the troubleshooting process. Seems GM designed the grid on the rear window for two purposes in our beloved Park Ave's. One is as a window defogger and the other is as an AM/FM Antenna.
If you haven't had any discernable loss of radio signals, the first thing to check is the 40A fuse labelled RRDEFOG located in the engine compartment, under the hood just behind the battery and to the left of the FUSE RELAY CENTER box. It's labelled FUSES. There are two snap retainer clips on the end of the cover. Carefully pry them away and pull the cover off. Inside the cover is a legend/layout showing where fuses are located. Look for the 40A RRDEFOG fuse, I think it's yellow. You can test it for continuity by using a DMM and probing the two exposed end points. With the DMM set on the low ohms scale you should read very close to zero. If zero, that means the fuse is good. If you want to be absolutely certain, you can pull the fuse out and probe the two legs. If you pull it out and it's bad, you'll know it just by looking at it.
The next point to test is located under the back seat. To remove the seat, push back on the lowest point of the front of the seat while pulling it up until you feel it release. There are two catches about 18 inches in from the doors under the seat that the seat latches to. Once released, lift it up and allow the two back catches to drop out from their retainers that hold the back of the seat in position. Remove the seat from the car.
Located close to the center on the floor you will see a black box with a star shaped spinner nut holding the cover on it. I think it's labelled FUSE RELAY BLOCK. Loosen the nut and remove the cover. Once again on the inside of the cover you will see a legend/layout of the fuses and relays. The relay you are looking for is labelled as HTD BK LITE, should be Number 1 and is the largest one of the group. Mine was made by Siemens. A quick test to see if the relay is functioning is to activate it via the dashboard control button with the ignition switch on, radio off, car not running and doors closed. Let everything settle out once you turn on the ignition swich before activating the control button. You will be able to hear a clicking sound as you activate/deactivate the control button. You can actually feel it, if you can reach back and touch it while turning the control switch on and off.
If the HD BK Lite relay, aka DEFOGGER relay, seems to be functioning the next step is to check for voltage feeding the DEFOGGER grid. To get to this test point you have to remove the Sail Panel (trim panel covering the drivers side of the rear window). To remove it, I started by pulling the slender part of the trim over the rear door out. It's retained by plastic tongues going into pressure clips. Pull it straight out so you don't break the panel or crack the tongues. The end of this this portion dovetails into a joint at the junction over the center post between the doors on the drivers side. Once this section is free, work your way back carefully prying and pulling. The main portion where the seat belt comes through is held in by press in plastic screws, which can be pryed out with caution. Once these are free you will have to pull the main sail portion up and forward as it has an interlocking leg that engages the back deck to keep the bottom in place. Once free, carefully work it down the seat belt and push it out of the way. You should now have access to the connections for the DEFOGGER grid.
There should be a black plastic module known as the Radio Antenna Module exposed with a two white wire connector and a coaxial antenna cable plugged in on one side and a two wire (black and white) connector plugged in on the other. The single connector side of the module should have the black and white wires going to the grid on the rear window. The white wire is for the antenna signals and the black wire is for the 12 VDC power to the DEFOGGER grid. With the ignition on and the dashboard control button activated, you should read between 11-13 volts on the DMM probing the black wire as positive connected to the grid and the DMM common lead to the car's chassis ground. If not, disconnect the two white wire connector on the other side of the module and probe the lower side of the connector, the side with the retainer, do not probe inside the module. You should read 11-13 VDC there. If not, the HTD BK Lite/DEFOGGER relay is bad, even though it's clicking, the contacts may be misaligned or pitted.
If you observe voltage on the lower side of the input connector but don't see it when you probe the black wire connected to the grid, reconnect the input connector and remove the output connector (the two wire black & white one) and probe the lower contact "inside" the RADIO ANTENNA MODULE. If you observe voltage there, then the RADIO ANTENNA MODULE is good. Look very carefully at the black wire going into the back side of the output connector to the grid and give it a good tug. There are signs and indications that GM may have had a bad rash of these connectors installed in vehicles where the crimps on these wires ended up on the insulation instead of nice bright copper strands and over time corrosion sets in and results in the defoggers not working.
If you found corrosion in this connector, there are replacements available for $4-$5 that can easily be cut and crimped in. I would also check all other connectors and crimped connections because crimped connections that have insulation under them created high resistance joints that cause more current to flow and create heat that melt and weaken other wires and connectors in the circuit. Check it all out thoroughly.
If your radio reception went bad at the same time that the DEFOGGER started to malfunction, I would suspect the RADIO ANTENNA MODULE and would have recommend you replace it but a wire connection is easier to repair/replace than a module.
Please let me know how you make out and I hope that this helps others with the same problem.
Tuesday, December 30th, 2008 AT 10:50 PM