I purchased the same napa plug in harness and plan to use the connctor to hook up my powered isolator box which I installed long before I knew about the napa part. Right now I have the isolator box connected to the tail light harness with those piggy back connectors, never felt very comfortable with them.
The disadvange of the napa connector, like most tee connectors, is you use vehicle turn signal power to run the trailer lights. Also if you back a boat into the water you are at risk of messing up your chassis wiring if something goes wrong with the trailer in the water. So if you do tow a boat remember to unhook the harness before you back the boat into the water.
If you plan to tow a utility trailer with just brake and tail light use the NAPA piece, you will be very happy!
To find the tail light connector just pull out the tail light then follow the harness, you will find the connector clipped to an interior structural member. Just lift up and pull the connector to the opening. Then disconnect, insert the NAPA converter, reconnnect and hook the connector on that interior structural panel.