Here is tonight's stock wiring enigma -or a case of me overlooking something. A large black/red wire comes out of the firewall on the driver side and goes around the front of the car in the front ('F') body harness. On the driverside a lead branches off and goes to a subharness that connects to the starter. This is what triggers the starter to turn. Back at harness 'F" the main black/red wire continues around the pas side fender and goes into the firewall. There the wire connects to the ECU and something else. My assumption is that the ECU uses it only to determine when the car is starting.

1. Why did they use such a large wire to connect to the ECU? It looks more like a power wire than a signal wire.
2. what else does it connect to under the dash? The diagram doesn't show anything, but the wire I have has two ends. Unfortunately The other end was removed a long time ago, so I have no memory of what it went to