I have an 88 Cheyenne 2500
Water leaks in front floorboard and can hear water sloshing under dash.
Any ideas

Take a long pipe cleaner or wire and run it up the air conditioner drain tube, which is located along the firewall on the passenger's side. Jack the front end of the car up and be prepared for a quart or so of water to gush out. The water has condensed from the air and collected beneath the evaporator coil.