Installing Electric Brakes on a PUP

Are you describing an existing four-flat trailer connection? If so, all that does/will ever do is control lights on the trailer. You'll need to move to a bigger connection- seven-wire is usual although you can get by with six if all you want to add is brakes and charging line.

The brake sensor doesn't connect to the brake lights. It requires a new wire all its own, and must be controlled by a unit installed inside the tow vehicle.