Adding a new car stereo to a vehicle's existing wiring is not as daunting as it initially appears. Within each vehicle, wire harnesses contain individual leads for specific functions such as speaker output, power antenna activation and conventional power and ground among others. Matching these leads with the wires from a vehicle-specific wire harness adapter lets you connect your new stereo correctly to the vehicle with no guesswork. Even vintage vehicles without a matching harness can be wired up using the same process.
To access the stock radio harness wiring, first acquire a vehicle schematic to identify and locate each screw, bolt and clip securing the radio trim bezel to the dash. This must be removed to gain access to the radio's mounting hardware. Using the correct screwdriver, remove these screws and place them in a safe location, like a magnetic hardware tray. Keep the screws segregated to prevent confusion as to which hardware fastens which portion of the trim. Gently yet firmly remove the trim, revealing the radio bracket. Remove this hardware and extract the stereo from the dash cavity. Disconnect the radio's harness from the one leading into the dash, along with the antenna lead.
Most aftermarket radios have a wire diagram printed on a label affixed to the top of the unit. If not, consult the unit's manual for cross-reference. Examine the package of the aftermarket antenna adapter, which details the function and color of each wire in the harness for your specific vehicle. Also note the presence of printing on each wire in the radio's harness and on the adapter. The harness that came with the new stereo and this adapter harness are the two you are splicing together to link the new head unit to the vehicle. If you have an older vehicle and no aftermarket harness adapter is available, remember that in nearly all vehicles the red wire is switched power, yellow is constant and black is ground. Solid blue wires are usually for the power antenna, while the blue wire with a white or black stripe is for accessories like amplifiers. The orange wire controls head unit illumination, based on the position of the headlamp switch. To identify speaker wires in an older car, hold one of the wires in each speaker wire pair against the bottom plate of an AA battery. Tap the other wire to the upper plate. You will hear a "pop" at each contact from the matching speaker. Label each one with a piece of masking tape and a pen.
Pull off the prestripped insulation ends on both harnesses. Twist the individual strands on each wire to prevent an accidental short circuit. Insert the matching wires into opposite ends of insulated 16-gauge crimp or "butt" connectors. Secure each crimp with a crimp tool. Insulate unused wires with a crimp-on bell cap. Note that it's not unusual to have unused wires or empty slots in the harness adapter, since vehicle manufacturers often share installation parts for head units with different features. After each crimp is complete, secure the individual wires in the completed harness with 4-inch plastic zip ties. Snip off the excess tail on the ties with a wire stripper or the end of the crimp tool. The wiring portion of the install is complete.
Other Steps and Accessories
To get most aftermarket head units to fit into your dash, you need a vehicle-specific dash kit. This allows the new stereo to slide into the dash like a stock unit. Slide the mesh cage off the chassis of the new head unit. Slide this into the opening of the dash kit. Bend up the four triangular tabs at each corner of the cage, securing the piece in the dash kit. Slip the new radio rear-first into the cage, until the tabs on the unit engage the catches on the cage. Connect the radio's harness to the back of the unit. Snap the larger end of the bundle into the matching vehicle harness. Slip the antenna adapter for your vehicle over the stock lead and then into the head unit. Push the entire assembly into the dash cavity, gently moving wires out of the way of the unit as it pushes in place. Secure the radio and trim in the dash using the stock hardware.
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