tyler: what year mustang?? There is a TSB on a bunch of 99 models fords for this intermittent wiper problem. (my dw has a 99 sable). Calls for replacing the MFS (turnsignal/wiper switch) with an improved part number. Of course the part is 92.00 (75 from Rusty) and now I am out of waranty, to bad I didn't know about the tsb when I took it in under warranty and the dealer couldn't duplicate the problem.
JP: I took my wiper switch on the sable apart and cleaned th contacts inside by taking a small pen knife and popping the cap off the turn signal switch, taking it apart, cleaning with alcohol, and putting back together. Not hard to do, just be very careful, there is a small pin in there that if you lose it, you are screwed. You could have a small piece of crap inside the switch not allowing it to shut off.