Guess I should have looked first. The IAC is under the air intake hose and yeah it has a large black plug. Has the two small coolant lines going to the intake next to it too.
Info on checking your engine codes is in the FSM along with testing methods for the IAC if you want to remove and test it as well.
Recently my idle was temporarily erratic when I was refilling my coolant after doing a system flush. That's why I asked if you checked your coolant level. If the temp sensor is getting bad readings it can effect the idle.

1990 LWB RS A/T
1990 LWB Base A/T
1991 LWB LS A/T Going...Going...