Wireless Backup Cameras, most work like this.
The receiver/display is usually always on (when plugged in to the cigar lighter)and receive all signals in a frequency (that's why they show other views occasionally if another car is close by).
The camera end consists of a camera and a transmitter. These are usually connected to the hot wire going to the backup lights. Therefore only transmitting when the light is on. To make this type work full-time you need to find a wire that has power on full-time, like the one powering the radio. Then run this wire to the camera/transmitter.
Hope this helps.