A car stereo is supposed to have two +12V input wires and one ground wire. The yellow wire is named "constant" or "battery". The red wire is named "accessory"

A car stereo is supposed to have two +12V input wires and one ground wire. The yellow wire is named "constant" or "battery". The red wire is named "accessory" or "switched". The black wire is ground. When the car is turned off, the red wire drops to 0V and the radio turns off. The yellow wire is supposed to remain at 12V to maintain memory state. The yellow wire draws a very low amperage, and would take years to drain the battery.