Wiper turn on automatically

I have 1997 Caravan. My wiper turn on automatically since new. Dealer can not duplicate this problem under warrenty period. Right now, this problem getting worse. The wiper turn on anytime, on any situration. Anybody have this problem and solved? Thanks.

Once this occurred to me and on my 90 Caravan, and I was so surprised to find out it was due to low wiper fluid in the rear door...don't ask me why but it caused my wipers to come on...