If you're certain the low-speed fan is not coming on during normal operation, but it does turn on when you jumper the relay contacts, then I would try replacing the low-speed fan relay (green). Maybe it is not closing the circuit when the computer tells it to. Not sure how to test it - presumably if you knew what the signal was that the computer sends to it, you could apply some batteries to the proper terminals of the relay and listen for it to close, but I don't know that info.
Here is a quote for the part from Bill Jenkins
Low speed relay
F5RZ-8B658-A List $34.92 Inet $20.70
Or it could be a harness problem between the computer and the relay. Or it could be the computer.
I just ordered my ECT today, now you are scaring me that it isn't going to do the trick.
