Given the history of this product (early 1990 Mitsubishi
products) I expect it is the engine ECU, located behind the radio
. You'll need to remove the two pieces of carpet trim/panel off of the tunnel, pop off the plastic caps (one on each side) and remove the phillips head screw. The trim now loose will in-hook from a point near the fire-wall and then back by the shifter. Looking behind the dash in the middle the ECU will sit vertically and the (3) yellow electrical plugs within it, unplug these and remove the three bolts/screws holding it in place (if my memeory serves they are No. 10 metric) Note the MD number on the ECU this is important as you must keep this common with the replacement. The death of these ECUs are the (3) capaciors within them. If you like you can open up the ECU and look for these; under them you might notice a thick grease like substance, this is the failure of a leaking capacitor. It is possible for you to replace these capacitors (must be done with care) If you purchase a rebuilt unit the
new capacitors are of better quality and less prone to leaking.
The attached link should assist.
I hope this may have assisted you,