Polk PA660 Auto Detect Turn-On



The literature for this amp says the speaker level inputs will trigger the amp to turn on when a signal appears. I've got a 12V lead to the remote signal port, but I must have used a memory lead as the amp is always on, regardless of the head unit. I'm using speaker level outputs from the stock radio also.
When I disconnect the 12v REM lead to the amp, the amp will no longer trigger on, no matter how hi I turn the volume up.
What am I missing?

Thanks in advance!