Start car engine.
Turn radio on.

What is the voltage at pin 5 of connector A on the amp? That line HAS to be 'high' to make the amp turn on.
I do not know how 'high' it must be, if it is at 12 or 13 volts, that is good. If it is over 5 volts that is PROBABLY good enough.
The manuals just don't say, but I DO know it has to be 'high' for the amp to turn on.