So just like the factory JBL amp the Rear DVD ECU receives a signal from the factory HU to power on/off. On top of that there are signals sent back and fourth allowing the DVD to play radio stations or cd's in the Factory HU, and to send the audio from DVD to the HU.
Plain and simple, the dvd ECU is powered on by the factory HU.
There is out there a module that allows you to still use the factory amp with an aftermarket HU sold my metraonline,
Metra Online
There might be some way to make it send the signal to the DVD ECU to power on which is what it does for the JBL amp.
Hope that helps.