Car interior lights have a few modes of working. You can turn them on voluntarily with the switch whenever you want. Or if they are in automatic mode, they turn on and off with the opening and closing of the doors. But if you have a custom light in the car, it is a different story. You can connect them with the lighting circuit of the dashboard switches if you want them on with the headlights. You can also install them with separate switches as well. But in modern cars with power saving mode, all the electrical outlets turn off when the car is parked. So, you don’t have to worry about starting the engine when you come back after a while.