Sometimes it's easier to pinpoint sound source by holding a piece of ear to your hose.
I'd do that with the relay box under the dash on the driver's right footwell area, behind glovebox, and behind radio... and think about pulling the battery saver relay to see if it stops, and if it does, swap that with another relay.
P1000 just means drive cycle not completed since the battery was disconnected. I don't have any particular reason to suspect them, but would also wonder about the door and back hatch, latches and switches.
One of the following may be the driver's footwell relay locations for a '99. If it is either of these, please let me know so I can add that info to the file name to ID it again later.
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