Disconnect the battery, pull out on the knob until it stops -- then reach in to the top of the headlight switch, push down on the little button, pull on the knob and the shaft will come out of the switch.
Here's a thread with a pic of what you're looking (feeling) for.
http://67-72chevytrucks.com/vboard/s...d.php?t=374643
EDIT: Dang, I type slow....