Hmmm, I kinda have a similar issue on my 2000. Every once and a while out of tue blue the wipers will turn on once, when in the off posiion. They work fine when I need them too. Its so intermittent that I've not found it necessary to figure it out. However, in your case it seems like it could be a faulty wiper motor.
2000 Mustang GT-Former giggle gas addict
1990 Mustang GT w/ Vortech V3
***resident gun idiot***