Two problems related to turn signal switch have been reported for the 1999 Ford F-150. The most recently reported issues are listed below.
The windshield wipers come on while the switch is turned off. The problem is an off and on issue. The problem started on July 20, 2005. Sometimes when turning the turn signal light on it will happen, and sometimes when turning left they will come on. Over the phone a dealership told the consumer that they thought it was due to the turn signal.
While driving in the rain windshield wipers stopped working, causing loss of visibility. Consumer states the multi-function switch was replaced.