How to Wire Amps to a Car Radio

By David Lipscomb

There are many factors that work against quality car audio. The biggest culprit is road noise, competing with and canceling out critical audio frequencies. As this noise forces you to turn the volume up, you'll quickly find that your in-dash radio's small amplifier runs out of steam. The solution is to add an amplifier, dramatically infusing your speakers with plentiful, clean wattage.

Parts and Preparation

Before removing even one screw or panel from the vehicle, assemble all of the tools and parts you'll need for the project. You'll need Phillips, Torx and Allen screwdrivers of varying sizes, so a universal screwdriver with interchangeable tips is a good idea. Wiring tools include a socket wrench and inserts, combination wire stripper and crimper, self-tapping sheet-metal screws, cordless drill with Philips-bit attachment and rough-grit sandpaper. Installation parts include an RCA cable long enough to reach from the radio to the designated mounting location, insulated crimp rings, insulated T-tap connectors, 4-inch plastic zip ties, 4-gauge power and ground wire, 16-gauge primary wire and 16-gauge speaker wire and inline fuse holder with fuse.

Preliminary Steps

Examine the internal fuse values of the amplifier. Purchase a fuse to snap into your inline fuse holder matching this value. This fuse protects the power wire leading through the vehicle, preventing a fire in case of a short circuit. Locate a mounting spot for the amplifier away from foot traffic and seat rails and providing at least 3 inches of space around the unit for ventilation. Locate a wire bundle under the hood that leads from the engine bay into the cabin of the car, and find where it comes out under the dash. You'll use this for the power wire. It's also a good idea to secure a schematic of the vehicle, detailing where each screw, panel clip and wire is located in the car. These are available online or from your dealer.

RCA and Remote Leads

Consult your car's wiring schematic to locate the screws holding the radio into the dash bezel. Remove these screws and set them aside in a cup holder or a zip-top baggie. Carefully pull the radio from the dash. Locate the RCA output jacks on the back of the radio. Slide the RCA cable into the back of the stereo from behind the radio cavity. Remove the side sill panels securing the edges of the carpet to the vehicle. Feed the RCA cable through the wire channel in this gap to the chosen amplifier mounting spot. Locate the blue accessory wire leading from the radio's wiring harness labeled "ACC." Cut a length of primary wire long enough to reach from the amplifier's mounting location to the accessory lead. Strip 1/2 inch of insulation from both ends of the primary wire. Crimp the spade portion of the T-tap connector to the bare wire. Clip the hinged portion of the connector over the ACC wire leading from the radio. Slide the spade into the slot on the hinged connector, then route the wire along the same path taken by the RCA cable.

Power Wiring

Open the hood to the vehicle. Remove the nut securing the black ground wire from the battery. Remove the nut holding on the red positive lead, but do not remove the wire from the battery. Cut a length of power wire that can reach without tension from the battery terminal to the amplifier's mounting location. Cut a smaller length less than 18 inches long. Strip 1 inch of insulation from both ends of the shorter cable and one end of the long lead. Crimp a ring terminal to one end of the short lead. Slide the ring over the post holding the positive battery lead, and secure the nut holding the leads in place. Insert the other end of the short lead and the stripped end of the long lead into opposite ends of the fuse holder. Secure these wires by tightening the Allen screws inside the holder over the wire strands. Slide the other end of the long lead through the grommet protecting the wire bundle you identified during your preparatory steps. Pull the power wire through carefully. Route this wire under the opposite door sill used for the RCA and remote turn-on lead to the amplifier's mounting spot.

Ground Wiring

Locate a section of sheet metal within 18 inches of the amplifier's mounting location. Sand the paint at this spot down to bare metal until it is clean and shiny. Cut a length of ground wire 18 inches or less. Strip 1 inch of outer insulation from each end of the wire. Crimp a ring terminal to one end of this wire. Screw the ring terminal to the bare sheet metal using a Philips self-tapping screw. Do not use a seat belt bolt or a welded location for the ground, since these do not provide proper continuity and may cause ground-loop whine or amplifier malfunction.

Speaker Wiring

Cut two lengths of speaker wire sufficient to reach the amp from the back of the radio. Identify the speaker wire leads on the radio harness using the label affixed to the top of the unit, or in the owner's manual. Snap a T-tap over each speaker wire. Strip 1/2 inch of insulation from both ends of the four conductors on each speaker wire. Crimp the spade portion of the T-tap connector to the radio end of each stripped wire. Slide these into the slots at the base of the T-taps. Route the speaker wire from behind the radio through the wiring channel occupied by the RCA and remote turn on leads to the amplifier.

Final Steps

Screw the power, ground and remote wires to their respective terminals on the amplifier. Screw the speaker wires into the amplifier terminals, matching the polarity observed at the radio. Slide the RCA cables into the matching red and white amp inputs. Screw the amp to the intended mounting location. Reinstall the radio into the cavity, reversing the steps taken to remove the unit. Snap the sill plates back into place, covering the wires leading to the amp. Secure the under-hood fuse holder to an existing wire bundle using a pair of 4-inch zip ties. Snip off the excess with the tips of the wire strippers. Turn on the car and stereo. Turn the amplifier gain all the way down. Turn the radio up to the 3/4 volume point. Turn the amplifier gain up to the point where audible distortion is heard, then slowly reduce the gain until it disappears.