I wanted to update this with my experience in attempting to do this with my 99. TLDR; I gave up and went with an SLA-22 harness that had come up for sale. However, I feel I was very close before giving up.
I followed the post above, but instead of attempting to wire up all 4 speakers, I was attempting to wire it up to just the front left and front right speakers. I also did some wire translation between the provided wiring diagram, and my attached 99 wiring diagram, but it's been a minute and I've forgotten the wire colors/pinouts. I had gotten it to a point where my unit powered on fine, but the output of the speakers was INCREDIBLY quiet at max volume. At this point I had put a ton of time into it, and an SLA-22 harness came up for sale so I jumped on it.
When I was installing the SLA-22, as I was reading it's manual, I saw it mentioned this blue-white "amplifier remote" wire, which I had yet to actually touch. The repair manual lists a L/W wire running from the radio to the amp as "MUTE", but I didn't actually know what to do with it. Is it expecting battery voltage? Does it just need continuity to ground? Whatever the case, I really feel like this was the last piece of the puzzle for me before I gave up.