[atlanta1]

2005 Mercury Grand Marquis, windshield wipers not working on high but will on low. Fuses are fine.

[CaptainRich]

2005 Mercury Grand Marquis, windshield wipers not working on high but will on low. Fuses are fine.

The switch is the likey problem here. Check for continuity through the switch when in the high select position: there should be ground on the Yel w/
Red wire and the solid Red wire, when the wiper switch is set to High and the wiper's operate properly in Low. If Low is working, that eliminates the relay and the harness leading to the relay.
Try plugging in a replacement switch and testing functions.
I hope this helps.