If anyone is interested, here are the pinouts for the 2000 SE stereo, the one with the tape deck and single CD.

http://www.se.mediaone.net/~rkrenn/StereoRear.gif

The REQ-Sig in, RX-Sig in and TX-Sig out use an "N-Bus" to communicate. I haven't found any doco on the N-Bus.

BTW, this diagram is from the Service Manual for the Stereo.

Richard