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Connecting your speakers to an amp
Tip: You can use your vehicle's factory wiring
Getting signal to the speakers
Sending a signal from a receiver to an amplifier's input via RCA-type cables or the speaker-level outputs is straight forward and simple enough, but getting the amplified signal back to the speakers—well that's another matter. The speakers are usually in four different corners of your vehicle, and running separate new speaker wires to each presents many challenges, including how to get the wire past the car door hinges to the front door speaker locations. A convenient way to send the powered signal from the amp to your speakers is to run the speaker wires to the harness that’s behind your receiver, where all your car’s speaker connections are accessible in one place.
Here's how you can do it
Normally, the speakers connect to the receiver through the receiver wiring harness behind the dash. You cut each speaker wire from the receiver's harness and reconnect it to a speaker wire coming from the amplifier. Then, the signal can flow from the amp to the speakers by way of the vehicle's original factory wiring. For 4-speaker systems, you'd need to make eight connections—a positive and negative wire for each speaker.
This means, you just need to run the eight speaker wires from the dash to the amplifier, just like you do the RCA cables. In fact, when you're installing the amp, it's smart to run them all at the same time.
New speaker wires from the amp connect to the speaker wires in the stereo adapter harness
to help you do it
Crutchfield carries a small selection of highly conductive, pure copper speaker wire, available by the foot, that will work well for sending the amp's output signal to the factory speaker wires.
A multi-pack of Posi-Products Car Stereo Connectors could come in handy here, so you can make all the speaker connections without having to solder anything.
High power amps need bigger wires
For amplifiers with over 75 watts RMS of output per channel, it may be better to go ahead and run new 14- or 16-gauge speaker wires directly from the amp to each speaker.
Factory speaker wires are very thin, with high electrical resistance, so they can cause noticeable power loss when higher wattages try to get through. But amplifiers of 75-watts or less aren't really affected by this, so running their outputs through factory wiring remains a practical and convenient solution.