That's a helluva find. Hopefully this may be your solution, although a time consuming one. Bookmarked that one.
If you look at this diagram, that Vigman posted a while ago, it appears that the only connection to the ECU is at pin 53. This diagram appears to have the speedo pin numbers for vehicles without air bags.
The other connections at the speedo are---
L (blue) off of speedo pin 16 to speed sensor [pin 14 on the online service manual (OSM) for with air bags
Y/G off speedo pin 17 to ECU pin 53 [pin 15 OSM with airbags and it feeds to the ASCD not ECU. ASCD looks independent of ECU in OSM]
Y/B off speedo pin 18 to speed sensor [pin 16 OSM with airbags]
B off speedo pin 14 to ground [pin 22 OSM with airbags]
14 and 18 are tied together, so 18 would be the grounded wire on the sensor.
Power to the speedo comes from fuse 10, 2D on the diagram to pin 24 [pin 29 OSM with airbags]
ECU pin 7 goes to the tach (I assume off ECU signal generated from CAS).
I've read in past posts that the ECU may have something to do with conditioning the signal to the speedo. I'm ready to be flamed but I don't see any feedback wire from the ECU to the speedo itself
(unless pin 53 is it based on other inputs at the ECU, which wouldn't be a conditioned speed signal back to the speedo), so my theory is that the two wires to the sensor off 16 and 18 would indicate you could test the speedo itself by taking the sensor out to the dash and running some wires off it to 16 and 18, spin the sensor and see if the speedo moves. Don't know if you trust this theory before getting any other comments on it or if you're brave enough to try it. It may prove the speedo works or not.
Which pins at the ECU are you checking for continuity with the speed sensor? That might just blow this theory up.