carpet (for most cars)
remove the drivers or passenger side seat. which ever side you're NOT running the power on.
remove the plastic door sill trim
then you'll see the plastic clip that holds in that side of the carpet.
lift up on the long clip, and you're done.

i suggest that you don't completely rip up the carpet, because getting it back the way it was isn't that easy. just release enough of the carpet to fish line the RCA's through. you're going to have to do some reaching and use the holes in the rug (where the seat mounts over) in order to help run the RCA.

back seat and trunk (for most cars)

lift up the carpet and run your RCA.
check the back seat for any sort of release pully. imagine if someone was sitting in the back seat, find the area between their legs where the seat meets the vertical carpet. look for a plastic release and pull up. (i've never done this on a 5th gen before, if there's no release, just tug up on the rear seat, it should be held in with a clip.)
then getting into the trunk is easy when the rear seat is up.