One last thing I just thought about today. It looks to me like the exact same data and clock lines that connect to the ECU are paralleled also to the A/C, AT, HICAS and airbag controllers. It must work that the interfacing device has to send a message to the car saying essentially "Hey ECU, I want to talk with you, everybody else shut up." I'd guess that it is possible that a fried data line on any single one of the controllers could prevent any communication at all. If that is so, it might be a good test to temporarily unplug all the other controllers an see of the ECU responds. If that works, plug the others back in one at a time to try to identify the culprit.
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