Where is the onboard computer on a 1991 Nissan Stanza?

Try looking under the front passenger seat. If not there most likely at the front of the console under the dash. These are the two most likely places in your car. There are two panels one near the gas pedal on the drivers side and one on the pasengers side. Take theese two panels off and the ECU ( brain ) is easily accesible without taking the dash apart. The bolts take a 10 mm wrench.