I'm aware that a lot of modernish (early 90s) cars with cable driven speedo drives have a signal generated from the dash, back to the ECU for SPD signal.
With the adapator thing I got from pick a part today, once I pulled the back off, it's just an angled flexible cable inside it, not geared or anything. So no way you could have two outputs off it.
Here's the information that I'm going off, and why I bring up cruise control....
If you dont have a SPD signal from the head unit, there is a very simple fix. Im not 100% sure if you all got it in NZ but here in the States, the 91-92 SW20's had a seperate speed sensor for the cruise control. We never seemed to figure out why toyota did it as it is redundent when the car already has a SPD signal from the head unit... Either which way, you can probably get one from the states for pretty cheap if they weren't used in NZ...
It goes between the speedo driven gear output on the trans and the cable (Threads on the the drive unit then the cable threads on to it)... It'll have 3 connects (+12v power, ground, and signal) obviously, +12v to ign. power supply, ground to chassis ground and the signal to the SPD signal pin at the ECU and problem solved....
The unit works pretty much the same as the magnetic REED Switch's in the Speedo head unit, but its electrical instead of mechanical (obviously).
93 and later cars all had electronic speedos the VSS (Vehicle speed sensor) opperates different on these. The VSS outputs a +12v digital pulsed signal. The ECU's including the Cruise Control, Enigne ECU, and P/S ECU (if you have speed sesitive powersteering) all look for a negitive pulse (pulsed ground). The Speedo Head unit conditions the signal (inverts it) for the ECU's to recieve the proper signal......