my defrost motor slows down when I idle at stops. Is my alternator not charging properly? 97 f150 / 4.6
thanks

 

Unfortunately I hate to deliver the bad news, but your hunch is about 80% correct. What it sounds like are the brushes in the alternator have reached their limit, and have worn down to far to contact the rotor's contact bands.
Essentially the unit needs replacement; which I would recommend be done a.s.a.p
Soon the connection will fail or become intermittent leaving you stranded.
This precludes all other possible faults such as slipping belt or pulley.
Take it easy