Have replaced the wiper motor and the wiper switch on my 1998 ford explorer and wipers still only work on high what next?

Try the multifunction switch (the turn signal switch on the left hand side of the steering wheel). This is a$65.00 item at the parts store or ~$45.00 on line. It takes ~ 20 minutes to change. Torx screws hold the unit in place. Phillips screws secure the cover.