Alright, I'm pretty sure I found that puppy. It _is_ under the dash on the driver side, pretty high up and towards the door relative to the fuse junction panel. About 8" directly forward of the headlight switch. The Instrument Interface Module itself is green, and it has a rather small 16-pin connector. The number and color of the wires matches the schematic in the service manual, so I think I am looking at the right connector. Thanks for the clues that helped lead me to it.
