Pin 4 or 5 from the MPI relay supply the CAS 12v, they're red just like pin 3 of the CAS. It also supplies the o2 sensor, injector resistor, all the emissions solenoids, ECU and majority of the engine. If you're only missing 12v at the CAS I wouldn't bother testing at the MPI because your open is somewhere well down the line from there.

I posted these a while back in https://www.dsmtuners.com/threads/1990-engine-ecu-mpi-circuit-diagrams.540595/ but perhaps you didn't find them.

You can see on page 73 that the MPI relay sends power when it's on to the ECU and to circuits off that page marked "A" which is on page 75. That's where it's shown that it goes to pin 2 (not pin 3) of the CAS (We are talking about a 1990 car right or have you converted to the 91+ CAS?)

It would be helpful if you had profiles for you vehicle(s).