If you're installing a car radio into a newer car, you won't have any problems getting power to the radio--there are power wires available that were used with the existing factory radio, and you can tap into these rather easily. But if you're adding a radio to a vintage car, or a vehicle that didn't come equipped with a radio to begin with, it may be necessary to look for another source of power. The car's fuse box is usually the best place to connect for the power your car radio needs.
Strip 1/2 inch of insulation from the red and yellow wires connected to the back of the car radio you'll be installing.
Twist these two wires together.
Cut a length of 16-gauge wire long enough to reach from the radio installation location to the fuse box. Take into account how you'll route the wire when you install the radio. Strip 1/2 inch of insulation from each end of the wire.
Twist together one end of the wire with the red/yellow wires you twisted together in Step 2.
Solder this wire combination together. Apply the hot soldering iron to the wires, let them heat up, and then touch the solder to the wires until it melts and flows into the joint.
Let the wire junction cool, then wrap it in electrical tape so that no bare wire is exposed.
Solder a fuse tap to the other end of the 16-gauge wire. Use the same method as described in Step 5.
Insert the fuse tap into an empty fuse slot in the fuse box.
This will provide your radio with a constant, unswitched source of power. Remember to turn your radio off when you park the car, or you risk draining the battery.