Auto need a power source ONLY on when car is off. So when car is on this source of 12V power would be OFF
A 12V, SPDT relay can do this. Use the relay's normally-closed contacts to connect the device to BATT when the key is off.
Basic idea:

ACC On: Relay is energized, disconnecting the device from BATT
ACC Off: Relay is deenergized, connecting the device to BATT

Schematic:



simulate this circuit &ndash; Schematic created using CircuitLab