Most good head units will have Front, Rear and Sub RCA Output. If yours doesn't then you might want to consider replacing your Head Unit.
"RCA cables
Now that you have your power and ground wires connected you'll need to give your amplifiers a music signal (usually the RCA cables run from the head unit). The best way would be if your head unit (radio/deck) has a separate set of outputs that can be used on the extra amplifiers. If your head unit is lacking these extra outputs then there are several other options.
If your amplifiers will be powering dissimilar speakers (subs, mids, tweeters, etc.) then you can use an electronic crossover to split and filter the signal going to each amplifier. Sub-bass frequencies would be sent to the subwoofer amplifier and high pass signals would be passed to the other amplifiers. The crossover will have built in line drivers which will increase the voltage of the low level signal. This can help lower the noise of the system.
Some amplifiers will have an RCA pass through circuit which can be used to drive another amplifier in the system. For example, amplifier A would have an RCA output for amplifier B. These are sometimes filtered which has the benefits of an active crossover without the extra equipment or installation expense. If you are using a filtered output make sure that it is adjustable or that it is at least at a useable frequency for your system design.
The least desirable method is the RCA cable Y adapter or splitter. This is a simple way to split a single RCA output into two RCA outputs. It’s basically splicing an RCA signal into two circuits. The drawback is that the signal voltage is also split so that each component is now getting a smaller low level signal which may increase the noise in the system." Source:
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