There's a switch diagram in this thread. Took me awhile to find it and I knew what I was searching for.....http://www.farmallcub.com/phpBB2/viewtopic.php?f=1&t=54960&hilit=headlight+switch+diagram
Depending on 1) which switch you have 2) whether or not you are still using a cutout instead of a voltage regulator 3) if it's been changed to 12V, will all have some bearing on how you hook it up.
For the complete wiring diagrams http://s101.photobucket.com/user/farmallbob/library/Farmall%20Tractor%20Wiring%20Diagrams?sort=4&page=1
Use #3 or #4 with a cutout. With a regulator use #6 which isn't for a Cub but follow the names on the wires and you'll be fine.
In the list at the very top of this page you'll see "manuals" twice. Click on the one to the right and scroll down to "How do I? resources links" and click on "Bob Melville's wiring schematics" will get you to the second link above. The first one is the result of a "search" using the Search in the upper right corner.
If you have the switch out of the dash I would use a short jumper wire to the battery and a light bulb and another short piece of wire. See if the light comes on when you turn the switch to D and gets brighter when you turn it to the B setting. You'll probably get a little smoke from the dimming resistor if it hasn't been used in a while, nothing to worry about. It will get hot so don't touch it.
An ohm meter would work checking between the input and the two light connections.
Welcome to the forum, don't hesitate to ask any questions if it's not clear.