Is it a harness from a new age WRX? The SMJ connects part of the ECU harness to the bulkhead harness that goes to the engine bay to the engine harness on the side by the battery. I think it has 40 or so pins and most of the non-shielded sensors and whatnot go through it. If you don't have it, it seems like it'd be awful lot of extra wiring work. Instead of just plugging that in, you are looking at connecting 40 wires manually...
Looks like this, courtesy of SCC's swap writeup: