I'm attempting to install the glass etc in my head light housing but can't figure out how the "spring" goes in to secure the glass or whatever it is intended to do. It's not in the "Ford Service" manual. Any help will be appreciated.

Remove the reflector then install the spring in the grove with the points facing out. Then push the reflector in with the points facing the lens then turn to lock in place. Then install the lens and ring and turn to lock. Don't forget the rope between the reflector and lens.

http://www.mtfca.com/discus/messages/599638/631341.html?1459969233

This should help. The bent out parts of the spring do not go behind the locking tabs, they are turned so they ride behind the wide areas between the notches on the reflectors.

Those four "V" tabs in the circle spring, placed behind the reflector edges, very slightly push the reflector forward. The lens, held in place by the bezel/rim, push the reflector back. The combination (lens pushed forward by one and back by the other) helps prevent rattles and breakage of the lens itself. It also applies pressure to the slots that the bezel/rim slide into locking it in place in the little notches at the end of the slots, which is what keeps the bezel/rim from shaking loose and falling off while you drive.
Getting the spring ring to snap into the groove near the front of the headlamp bucket is the tricky part.
There should also be a gasket (string/cord is often used) between the lens and the reflector.