Pull up the angled toe board along the back wall at the floor level, which will require the use of a screwdriver with a #2 Robertson square drive bit. It is not a Phillips head. Find the wires you need to tap into in the wiring bundle in the "rats nest" under the toe board.
I installed a 4-Pin flat trailer plug in the rear by drilling a hole through the floor in the area under the toe board. I would recommend drilling a 3/8" hole from the inside so you don't drill into any existing wiring and routing the wires in from the outside. (The wires you want to tap into are shown below.)
Finish up by caulking the hole around the wires with a good grade Marine sealant when your done with the wiring. My recommendation would be Sika-Flex 231 or 3M 4200.
My 2008 has the following color coding for the rear wire bundle under the slant board, which I had to decipher wire by wire in the rear bundle under the toe board when I installed a 4-Pin flat connector to power the
lights on my Stow-Away box. The actual trailer wire colors are the far left column.
TRAILER----------------FUNCTION---------------------4-PIN CONNECTOR
WHITE-------------------GROUND-----------------------WHITE
GREEN------------TAIL/RUNNING/LIC.----------------BROWN
RED--------------LEFT TURN/BRAKE LT.---------------YELLOW
BROWN---------RIGHT TURN/BRAKE LT.--------------GREEN