What you could do to install this Surface Mount Back-up Alarm # BKUP-97 is install it inline with the purple wire of your 7-way. You would cut the purple wire and then attach one end of the wire to one of the contact studs and then attach the other wire to the other stud, and then when there is voltage on this circuit the voltage will run through the # BKUP-97 activating the alarm and then go to the 7-way.
I attached an FAQ article on trailer wiring for you to check out also. view full answer...