Go to 6066gmcguy.org under "Lighting & Electrical" then under "1964-1966 GMC Park Lamps Rewire." (sorry guys--it's just where i found the information. Pls dont shoot the messenger)

1) Unhook your battery
2) Remove your headlight switch by pressing down on the spring-loaded button underneath the dash and pulling on the headlight switch's knob/rod. (see website for more info)
3) Using a wide bladed screwdriver, unscrew the retaining nut/ferrule (or whatever it's called) holding the headlight switch in place.
4) The switch should fall down from behind the dash at this point.
5) Clip the wires going from terminals 7 and 4--leaving enough wire so that you can strip them and wire them all together. I had to use spare wire pieces to make these "leads" longer--not enough slack in the wire to do this with the original wiring alone. Chevrolet was being "thrifty" with its original wire lengths.
6) Reassemble your headlight switch via steps 3 and 2--in that order.
7) rehook your battery back up.
8) you should be GTG now.--your park lights will remain on with your headlights and your turn signals will work as well.

FYI, i messed up and cut terminals 6 and 4--then wired them together. that made my park lights turn on at the switch's first position along with the headlights. then at the second position, the headlights remained on and the park lights went back out. go figure... just another one of my screw-ups.