You don't have to install a car stereo into a car to make it work. You just need a power source. In the case of car audio gear, you'll need a 12-volt battery, like those used in car electrical systems. With the battery and a few simple parts and tools, you'll have the car stereo powered up and ready to go in no time.
Strip 1/2 inch of insulation from the red and yellow wires on your car stereo's wiring harness. These are the constant and switched power cables.
Twist together the bare ends of the yellow and red wires and slide them into an alligator clip. Crimp the wires into the clip with a crimp tool.
Strip 1/2 inch of insulation from the black wire on your car stereo's wiring harness. This is the negative, or ground cable.
Crimp the bare end of the black wire onto an alligator clip.
Connect the radio to the battery by first clipping the alligator clip with the positive wires onto the battery's positive terminal, then clipping on the negative wire to the negative terminal. Turn on your car stereo to operate it.
If it's necessary to have the radio fairly far away from the battery, extend the wires by adding on lengths of insulated wire.