Pull the battery and have it tested for a dead cell.
I hope you're not saying you have the original battery in that car. If so, just change it and go away. In a 97 Park Avenue the meter reading of "nothing" that you state is wrong I believe. Maybe you just left out some details. Modern cars always have some draw on the battery when the engine is shut down. It can be substantial for a few seconds to thirty minutes until the computer goes into sleep mode. Usually it's only a few seconds though. You'd see 50ma or so in sleep mode, but it varies. So your numbers, meter, and battery are suspect at this point.
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