Powerful custom car stereo systems require installation of additional amplifiers, which drive the speakers and subwoofers. To connect multiple amplifiers, you need at least one amplifier with RCA outputs to transfer the signal from the head unit to each amplifier. These outputs allow you to run an additional RCA cable between amplifiers. Wiring an amplifier with RCA outputs involves the same steps as installing a regular amplifier, except that you will be installing two amplifiers.
Attach the ground wires to both amplifiers. Screw the opposite end of the ground wires into the chassis of the vehicle. Inspect the part of the chassis to which the ground wire is attached, and clean off any paint, dust or debris.
Connect the speaker's positive and negative wires to the corresponding speaker terminals on whichever amplifier you want to power the speakers. The first amplifier commonly is used to drive the standard speakers, and the second amplifier drives the subwoofers.
Connect the RCA cables to the RCA outputs on the back of your stereo. Run the RCA cable to the first amplifier and connect the cable to that amplifier's RCA inputs. Connect the second set of RCA cables to the first amplifier's RCA outputs and the second amplifier's RCA inputs.
Connect the vehicle's stereo 12v accessory wire, typically a blue wire, to both amplifier's remote turn on terminals by running a wire from the head unit to the amplifier.
Run the power wire from the battery's positive terminal to the amplifiers' location in the vehicle. Connect the power wire to both amplifiers amplifier. Make sure that the wire is the correct gauge to power both amplifiers and that the inline fuse is rated for two amplifiers. It is better to run a single wire and split it into two separate wires to power the amplifiers, as opposed to running two separate power wires.
Disconnect the negative battery cable from the vehicle's battery while working to prevent damage to your equipment and injury to yourself.