Okay, a little more diagnostic info (Plus diagrams, hopefully this will help someone out one day if they have the same issue):
I ran down the wiring diagrams, according to these for a 99-00, the "data" should come from ecu pin 1D (Green/White wire), and run into the under-hood connector closest to the passenger side & rear of the car, and also to the OBD-2 connector in the cabin (also the Green/White wire):
I pulled the harness from the ECU, and tested the connection between the ecu and the OBD-2 port: No connectivity.
Then I tested the ecu -> under-hood diagnostic port, and there IS connectivity:
Last I tested the connection between the under-hood port, and the OBD-2 port: Also no connectivity
Then I ran a wire from the under-hood port, back-probed into the OBD-2 port and SUCCESS: my BlueTooth reader connects and communicates great with the ecu with the jumper wire from the under-hood to OBD port.
So... good news is: I've got a broken wire/connection, bad news is: I've got a broken wire/connection.
I traced the wire back from the OBD port, and the furthest back I can get is about 8 inches from the plug where it joins with a giant wiring harness and disappears beneath the dash to places unknown. Does anyone have any idea where this wire goes? I can't imagine it has much distance to run, considering it's about 12 inches from the ECU harness to the OBD port.
Again, I haven't touched a wire on the stock harness since I last used this BlueTooth reader about a year ago when it worked fine. I have no idea how just this one wire could've gotten broken, you'd think whatever caused it to go bad would've gotten the power/ground wires as well.
I guess I could hack it and just run a jumper wire, but I'd really like to figure out wtf happened and fix it.