Mine showed similar symptoms last year, then I didnt use them all summer. Went to use them this winter and no response whatsoever.
I just ended up replacing mine. There is a rubber boot/housing on the stock wiper motor, and once water/salt/etc gets in there, it just sits and rusts, esp if you are like me and leave the top off all summer so they dont get used. On the bright side, replacement is pretty straightforward and the new motor was only about $90 from autozone.