The small black box between the clutch/pulley and the compressor is the commutator brushes. If these do not touch the ring on the back of the clutch, there will be no power to the clutch, and it will not engage. This either needs adjustment, or replacement, or the clutch is burned out. Any Mustang dealer, Glazier's, etc., should have these now that you know what to ask for. A little cleaning or adjustment may put the one you have back in action, too.
