To maintain settings in car radio, stereo etc:
hook up a 9 volt dry cell battery as in smoke alarms or similar to you cig lighter and plug in immediately after disconnecting the car battery.
(Positive to positive).
If the items cannot retain memory even for a fraction of a second, rig up 12v instead of 9v, plug in the cig lighter connection and then disconnect the car battery very fast, or better, put in a diode to stop the car battery pushing current into the dry cell.