Hiya,
Yep the CAS is in the dizzy.
The control unit for the self diagnosis proceedure is behind the trim panel on the passenger side footwell. Remove the trim panel and you will see the control unit, if I remember correctly, you will have to remove the mounting screws for the unit and turn it round (as the lamps that display the fault codes are facing away from you) - but leave the harness connected.
There is also a diagnosis mode selector switch, which you will have to turn with a screwdriver.
If you do a search on this board, there should be a thread on here that lays out the self diagnosis prceedure.
Hope this helps
.
Or if you know someone else with an s12, I guess you could just try their dizzy on your car, to see if that solves the problem.
Let us know how you get on
.
Regards,
Ian.