Step-by-step Instructions for Wiring an Amplifier in Your Car The power wire from your amp wiring kit (usually 16-20 feet in length) needs to run from the battery, through your car's firewall, through the car's body to the amp. Find an unused grommet in the firewall or one that already has wires or cables passing through it and that has enough room for the power wire to Car Stereo Wiring Diagram -75% - Now: Car Stereo Wiring Diagram AdDiscover hundreds of ways to save on your favorite products. Get the Deals now!Without Auctions · Great offers · Only Auctions · Free Shipping