Why mostly in rainy days the windshield wipers on your 1998 Ford Ranger work only at highest speed with switch turned all way to bottom?

This is actually not true, at least based on the vehicle and wiper design of a 1998 Ford Ranger. The windshield wipers typically work at various speeds, including low, medium, and high, controlled by the switch rotation position. There might be an issue with the wiper motor, switch, wiring, or other components if you're experiencing unexpected behavior with your windshield wipers.