On your 99-04 Mustang, look where the ceiling meets the front windshield. Directly above your head is the upholstery. It extends to the sides and the edges are tucked under some trim. To the front, is it just an open seam?
Yes it is open...the roof upholstry is attached to what feels cardboard. You can bend it away to get access behind there. It is where I am currently running wires for my radar detector install.