I have done this before and started to type up a response....there is just WAY too much to type.
The fender, strut covers, turn signals, need disconected/removed. You need to splice the new taillight into the 'rear harness' that runs to the your stock taillight.
decide if you are bobbing the rear fender (most likely) and if you are running turn signals...etc....
Do you have a manual....? You can see how to disconnect all of the stuff and which wires to splice you new tail light into.
If you have a manual, I would be willing to talk to you on the phone about this. This evening.
I did this with a second fender, bobbed the end off where the tail light used to be. Cut the connector off that went to old taillight....spliced in the turn signals and new taillight. I also added a 'trailer connector' and switched the turn signals to red running/brake lights.