From what I read, the black box above the rear view contains a camera that reads and translates the moisture on the windshield. If you stand outside your vehicle and pass your hand over it, the wipers will go on.
I found that my headliner was not put back quite right after the Sirius install. The headliner, which is molded plastic, was not tucked back to where it was originally. The headliner was pushing against the camera unit (black box), and causing pressure against it, which would activate the wipers......All is well again in auto-wiper world.