With my wiring, there are are actually two 28 pin harnesses. My suspicion is that my my OEM headunit had a different pin which would trigger the 12v AMP signal. Then the adapter would route the wiring two the correct pins. I'll have to open it up again and see what I can find on the weekend.
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Edit.
My backup plan is to get a multi-meter and do a continuity test to see where the wire from my amp is ending to my OEM receiver harnesses. I've found this picture in a different topic (
https://www.ft86club.com/forums/show...1&postcount=25), so would I just check where the green wire is connected to?