I reckon its something with the start circuit.
if the factory wirings rooted, just ground the deck to the nearest screw, continue to use a constant feed from the battery, but with the switch wire of the HU, feed that from the constant too, but with a switch.

so you can flick it on or off.
or just feed both with a constant and use the HU on off button.
thats a pain in the arse but.

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Balls.