Depends on where it broke. If the knob itself is real wiggly, could be the spacer (bezel) that holds it to the dash. Take a look, and if that is the case, you'll have to remove the switch from under the dash (there's a little button on the side of the switch assembly, under the dash, that you'll have to push in order to release the knob shaft from the switch. Take a look at your manual to find it). After pulling out the knob shaft, replace the bezel onto the knob shaft and insert the knob shaft, with bezel, back into dash and reconnect to switch. This should hold your headlight switch in place.
If you need a new bezel, you can order from any ford dealership. And the Haynes manual, I found, has the best description for the headlight switch assembly.