My Daytime Driving Headlights (both) are Flashing off & on Repeatedly, Rapidly.

What is your question?

This always happens while running. whether parked & running or while driving. BUT, If I turn my headlight switch ON-Manually...The Headlights (both), stop flashing and stay on. "What can be wrong?" Are the headlight wire harness' Blown? (Says Manual) or what? How Much should a repair like that cost?