How do i put in another amp if my head unit only has 1 rca output?

It would be best to use an active line driver. Using y-spitters is OK but every time you split the voltage you decrease the intensity of the signal from the head unit to amp which degrades sound quality and can introduce more noise into the system causing total harmonic distortion to increase.

http://www.sonicelectronix.com/cat_i697_line-drive...

By the time you spend the money doing that, you can replace your head unit with one that has more RCA ouputs.

On the again of the top unit, there may be an output, you hook up a RCA cable to that output. The RCA cable runs to your amplifier, then you definately run the subwoofer wires to the amplifier. If you wish to add a 2d amplifier, which you can get a y-splitter or get an amp that has pre-amp outputs, so you could have a RCA cable coming in, and have an RCA cable popping out to wire to a second amp. Hope this helped:)

I would just buy a new head unit with more RCA outputs.

You can buy rca y splitters. depending on the quality the can be from about $5- 20. you can also use the rca out on your amp if it has some.

Use the output that ordinarily powers the door speakers. You could solder on RCA plugs.

you could simply use the speaker outputs from the head unit