I finally found the problem with the Impreza ... it's right in the center of this diagram. I'm supposed to have a connection which runs all the way to Canada, it would seem ... back to the drawing board.
Actually, I found another diagram which shows all of the connections to the ECU which are important. It gives values for them with the ignition off and on, so I'm going to go through each one, back probe the connection, and get the results. Hopefully I'll be able to see something which is amiss and get it going.