, powering down when ignition is turned off etc.

Wait a minute. Is it powering down when the ignition is turned off, or is it going into standby mode when the ignition is turned off?

If the Mk2 is not going into standby mode when the ignition is turned off, if it's fully powering down (probably accompanied by a pop from the amplifiers) then I continue to stand by my initial diagnosis: Installation wiring fault of some kind. There is nothing wrong with the player itself: it's getting power on the ignition line only and not the constant-power line.

If the Mk2 is properly going into standby mode when the ignition is turned off, then the wiring is probably OK and you should contact support.