Need some more info on what you are doing. I have seen alot of tow bar installs where the tail lights are just tied into the trunk harness and then oyu close the trunk on the wires when you tow. I actualy like this method for infrequent use, since it keeps the connector nice and dry in the trunk when not in use.
You sound like you are hauling a camper though. Does the camper have it own battery? you will need an isolator if you plan to keep it charged from the cars alternator. There are plugs under the carpet where wires can be pulled through for a permanant install of the BMW harness. Are you using a BMW towbar or an aftermarket one? BMW harnesss or aftermarket?