Do you know how to test for voltage drop across a line? You place one test lead at each end of the wire (the wire needs to be connected so there is voltage to read), place the negative lead on the side closest to the + terminal in the circuit then place the negative on the other end. The voltage you see reported will be the total drop across the wire. More than a couple hundred mV and the wire should be replaced.
When testing across a larger circuit it is sometimes easier to test the whole thing and then slowly close in until you've found the individual point of failure. So in this case you'd place the negative probe on the battery's + terminal. Then you'd place the positive probe at the harness' +B connector resulting in a reading of a drop of about 10v. Now you move back along the circuit to the next junction and test. If the voltage drop is still present you move along to the the next junction closer to the battery. If it isn't then you test the voltage drop across the wire at the ECU harness and that first junction.
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