I think what's needed here is a wiring diagram for his '88 car. I know my '83 car had a computer yet it had nothing to do with the AC. But on my mom's '89 the computer had plenty to do with the AC, so we may be overlooking something. Either way, with the dash switch, when AC is chosen 12 volts has to go to the pressure switch, and if it is good and sees pressure, then the compressor and fast idle solenoid get energized. That old diagram shows the heat/AC as a single fused circuit, but it may not be on his car.
