Been going through this crap myself (a common issue), doing a lot of research and it appears to have several possible causes.
(1) Like mentioned above, a dirty multifunction switch can do it. Here is a link I found that was helpful:
http://www.therangerstation.com/tech...ntomwipers.htm
(2) Use your owners manual ($.99-cent download at eBay if you don't have one) to locate the 2 wiper relays in the electrical junction box under the hood. Clean the contacts (I actually used sandpaper on them and this ended up being the culprit even though the contacts didn't look that bad) or buy new ones @ about $19 each.
(3) The GEM module (which on my 99 is located behind the radio console, other years different) has been known to have solder connections fail and can be re-soldered (mine looked fine, I had to Google several part numbers off various modules under the dash to make sure I had located the right thing).
(4) The wiper motor itself. It has an internal contact that controls the "parking" of the wipers. Came across a post somewhere that told (no pictures) of how to get at it for cleaning. Sounded too complicated for me and I just bought a new one...which was not my problem-solver)
Every one of the above has solved somebody's similar issues. Like I said, mine was the relay contacts. I would try that first (instead of last like I did) as it is the easiest, fastest, and a no-cost possibility. Then the multifunction switch.
Good luck