The black computer box above the passenger's toe is the ICU or Ignition Control Unit in an '89. If you click here: ICU (250K image) you can just see the ICU in the very right of the photo under the windshield wiper drive. Not a likely source of your idle speed problem, though, not impossible either, as the throttle position switch (TPS) that is a likely cause is connected there as well as to the ECU you replaced.

If plugging the idle bypass hose brings the idle to a lope, you have mostly eliminated large vacuum leaks. Check the opening of the throttle body again, make sure it closes without binding, then make sure the TPS clicks when the throttle is first opened. It should travel a visible distance open before the switch clicks to assure the contacts are actually closed (electrically) when the throttle is closed. If this is so, measure the switch to verify. They can swell up if carb cleaner gets inside.
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Art Benstein near Baltimore