Check your HU rem cable output.
1. If it has +12vdc, the output is ok.
2. Since you have changed the wire for the REM out. I am suspecting the low amperage output which is not enough to drive the latching coil for your amp to turn on.
3. Let the HU REM out to drive a relay's latching coil. (Pin 85 and pin 86 to GND on the relay).
4. Pin 30 to a fused Constant/ACC +12vdc and pin 87 to your REM in on your amp.
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Good luck.