Wiring into stock stereo for sub.

Any easy access to front speakers for high input. Or a harness add with RCA's? In which what's correct way to get to the haeness. And will that affect the warranty

 

First off I want to be clear I know next to nothing(maybe even less) about audio speakers.

The 10 speaker trucks come with a sub behind the left rear seat, if its like the F150s then its the only large black box back there and it has a four pin green connector, now because of how you are asking I would assume that you do not have the 10 speaker option but it is defiantly worth checking if that connector is present since manufacturers often use the same wiring harnesses regardless of option/trim level.
Here is the wiring diagram, the connector face of the sub connector, the location view of said connector.