Okay, PROBLEM SOLVED. Near as I could tell, it might have been the seatbelt shoulderbelt retractors vibrating, or maybe the third brake light vibrating, or maybe the thin sheetmetal of the rear deck vibrating, or maybe the speakers vibrating from being mounted sloppily by being drywall-screwed into the underside of the rear deck. (In short, I don't know exactly what the $#%@ was vibrating, but I fixed it. Here'* how:
(1) Removed everything. Rear seats, rear decklid trim, seatbelts, 3rd brake light, tarpaper seals, speakers,
(2) Cleaned the rear decklid of tarpaper residue with mineral spirits.
(3) Made new seals using Peel-and-Seal to replace the factory tarpaper, not including the sheet over the 3rd brake light.
(4) Ran a strip of foam rubber around the 3rd brake light where it meets the rear window.
(5) Bought a pair of factory 6 x 9 speakers and mounting brackets from a junkyard. Tossed the facotry 6 x 9 speakers, cleaned up the brackets, installed new foam rubber gasket on the bottom of the brackets, traced larger elliptical opening on face of the brackets and used a Dremel tool to widen the brackets to fit my Boston Acoustics CX93 6x9 speakers. Fastened the plastic retaining ring on the face of the 6x9 speakers with polyurethane glue. Installed the speakers into the brackets with factory screws.
(6) Sealed the entire rear deck with Dynamat Extreme. Yeah baby!
(7) Installed the 6x9 speaker and bracket assemblies using stock factory clips.
Encased all rear-deck wiring in split-loom tubing.
(9) Installed 3rd brake light assembly.
(10) replaced my 97 "back of the trunk" fiberboard piece with the nice plastic one that I snagged of the 94 in the junk yard. MUCH nicer!
(11) Cased all of the wires to the Alpine 4 channed amp in my trunk in split-loom tubing.
(12) Reassembled the seatbelts, rear deck trim, and rear seat.
Test drove: ZERO vibration, and much tighter, more precisely imaged sound from the rear 6x9 speakers.
STILL TO DO: I'm not going to put new tarpaper over the 3rd brake light. With Dynamat Extreme covering the rear deck, I just don't feel the need for more tarpaper. Tarpaper is just messy stuff. Instead, I'm going to blanket the 3rd brake light with a sheet of aluminized insulated sheeting. I'll tape it down or something. I just know I'm not going to use tarpaper.
When I do that, I'll take some pictures of it. I was just so busy today I didn't have time to snap any pictures.
I'm also going to get an active crossover to go in front of the amplifier to roll off the deep bass to the front speakers so they won't distort, and I may work out a way to add another amp and a sub somewhere in the trunk.