[scarceller]

So it's not the ignition switch.

Something else is connected on that yellow wire that's feeding just enough current to keep the solenoid energized.

Not sure about the 930 but the 3.2L cars have a heat control relay at the rear fuse area that is also tied to that yellow wire. If you have that controller try disconnecting it. Also lower the 2 heat levers near the hand brake and turn the heat system totally off, also totally turn off the AC and test again.

We need a clear wiring diagram for that 88 930, look over the schematic for anything attached to that yellow wire.