mitgiraffe

The check engine light on my 1989 Subaru GL has been on for the last several years. The car runs fine (relatively), but I would like to know what is causing the light to come on. I have tried to read the codes from the ECU, which I was under the impression of being the box right behind my back passenger seat (accessed via the trunk). However, I can find no lights or buttons/wires to manipulate to obtain the codes, so I wanted to verify that I was looking in the right place for this particular model, and if so, what am I missing with this unit? Thanks.