The sensor you're looking for is actually in the instrument cluster. The speedometer driven gear is attached to the transmission and spins a cable that goes to the instrument cluster. There, it works the speedometer and also Vehicle Speed Sensor 2, which converts it to an electrical signal sent to the ECU.
If both the speedometer and VSS2 are failing, I'd suspect the mechanical connection between the cluster and the transmission. The driven gear may be stripped or the cable may be bad.
There doesn't appear to be a speedometer cable attached to the transmission. Does the 90 model have a speedometer cable, and if so where does it attach. Many thanks for your help.
I don't know exactly where it is, but it is apparently attached to the differential case. I'll try to get the relevant scans from my service manual for you, but they're not all that helpful (at least to me).
