The auto-power-off feature works by monitoring the voltage on the power cord. If the CDR830 detects that the supply voltage is gone, then it assumes that the engine is turned off and it will automatically turn off after the appropriate timeout.
On some cars, however, the cigarette socket is still live even when the key is out of the ignition. The CDR810 will always see the supply voltage and it will not know that the engine is off. Unfortunately on these cars the auto-power-off feature will not work.