The early days of computer controls were kind of like the Wild West -- almost anything went. Prior to the debut of OBD-II in 1996, manufacturers put computers wherever they felt like, programmed them however they felt and used whatever connectors they got the best deal on. Those early computers weren't exactly marvels of engineering either, by today's standards. If it's come time to replace your 1980s-spec computer with something a little better or less broken, you can expect a slightly confusing hunt for the ECM.
If you've found the computer port under your passenger-side dashboard, you've probably already followed its wiring harness to the floor -- where it kind of disappears. While you might expect to find the other end of the wiring harness coming out under the car, it in fact terminates in a small cavity between the interior floor and the bottom of the car. Pull up the passenger-side carpet and padding to expose the metal floor beneath. On the floor you'll see a roughly square panel with rounded-off corners. Remove the four nuts from the recesses in the panel, and pull the panel up. In a protected compartment beneath the panel, you'll find the ECM module. It rests on the four studs onto which the panels nuts were screwed.
