A photo of what you're looking at would help. If you're swapping the housing only you shouldn't have to swap any harnesses (unless one is bundled with the new housing and you want to keep it).
Undo the 2 x screws. Whole housing slides out / unclips towards the rear of the car.
Then you should be able to remove the bulb holders and bulbs from the original housing. The wire harness could be snipped loose if it's tied to it.
The other end of the tail light harness goes through the metal body structure and into the inner trunk area where it connects to the main vehicle harness behind the plastic inner panels on either side of the trunk. It's pretty simple to get in there, but is a bit involved. This is why I'd say keep your original harness if the lights themselves are working fine.
Ok so I love this answer. The part I have is the whole part going down to the plug. On the part that I'm replacing I'm able to easily grab the bulb out but the other part really doesn't like me (I can't get it out.) So in your opinion I should cut those wires on the troubled part and put the new one in rather than plugging the new part all the way to the bottom? ( I still don't know how to access the plug )