This is the only C10 bodystyle left in ASA, due to the fact that nothing before 1988 is in the program (CAPS when it first came around, or maybe an older incarnation I don't know about).
Spacewagon C12. Shows the ECU shape and location in the dash area. That is the box you are looking for. Heck, it could be in the center console below the radio and on the floor board/trans/exhaust tunnel area, right side of the dash (like up the A-pillar but still inside the dash, etc etc.) Maybe someone like this guy would know: GoSlowTurbo. He is driving a model like yours, but likely with a different motor, so I wonder if the ECU is still located in the same area. When I stripped the two cars I bought, I thought I got an ECU out of each, but turned out they were trans control boxes and from the driver side of the dash area. I didn't know what the ECU's looked like back then.
I googled 1987 Mirage ECU using google images, and found this on ebay:
http://www.ebay.com/itm/87-MITSUBISHI-M ... 2a2358bdc5
Hard to miss this somewhere up in there, but they seem to tuck away nicely. Hard to say where they crammed it in.