Basic sub setup
 

I have a kicker amp and 10" sub that I was going to throw in the FRS. I still have the factory 2013 head unit. Should just be as easy as:
1.Run wire from battery to amp
2.Run RCA from amp to back of head unit
3.Run turn on wire from head unit to amp
4.Ground amp
5.Wire amp to sub

...right?

I've never had a car with an external amp in the factory system. That shouldn't effect anything having to do with the sub right?

If** the head unit has rca output, if not; you can use a line output converter(some have turn on wire built in).
Otherwise you are good to go.

If you have the 2013 with the shitty no screen head unit then you have RCA outputs.

If you have the 2013 W nav unit then you need to
1. Run 4/6/8 Gauge wire from battery terminal to amp.
2. Ground the amp to the ground point in the trunk
3. Tap into the left side and right side channels of the speakers. ( You can do this from the stock amp in the trunk). (check wiring diagram for colour code)
4. Connect the green wire from stock amp (in parallel) to the new sub amp. This is the remote turn on wire
5. Connect the amp to the sub.
6. Make sure your settings in the Headunit is set to external sub (check on the forums how to get to this menu)

Shouldn't have any problems with interference unless you short circuit the amp

I have the shitty one with no screen for now. I connected everything up but there is no output to the sub? The amp comes on when I turn the ignition on. But no sub. Is there an option on this head unit for sub output? I looked and couldn't find one.

I guess the RCA outputs on the back are just for show. The stereo shop threw in a high to low converter and everything worked fine.

If you want to use the RCA outputs on the HU, you have to go into the setup menu below, and turn the sub-w cntrl to "on", its off by default.
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[ame="https://www.youtube.com/watch?v=uthViYfSkMs"]Scion FR-S Radio Settings - YouTube[/ame]

What he said, they work, I have run a sub this way. Line out works too though.