A simple indicator with buzzer that can be used to warn you if you forget your vehicle lights on, can be designed using this electronic scheme.
With headlights on (S3 closed), the interior lamp L1, does not turn on until one of the front doors opens (where S1 is closed is either S2). Meanwhile, the buzzer is energized and sounds. It can be stopped or if you close the door that turn off lights (S3).
Diode mounted in series with the buzzer is necessary because, normally, point B is connected to +12V through L1, and point A on the ground, the lights (L2 - so, here is represented just one). In these circumstances, buzzer should not sound.