lights won't dim

Your light-dimming resistor on the back of your light switch is evidently open or one contact inside the light switch at the resistor is corroded, dirty, or broken. (but, first just check to make sure that the screws holding the resistor on the back of the switch aren't loose or corroded - careful, not too tight or somethin' might break). There were two versions of light switches used: One had 4-positions: L-H-D-B, and was used on tractors with a magneto and a cutout relay on the generator. That light switch had 2-resistors on the back. The light dimming resistor was the larger one (not the flat-wound one which controlled battery charging on the 6V cutout system) I'm going by the manual's pictures.

The second version of light switch was on SA tractors with battery/coil ignition and a voltage regulator. This switch had 3-positions O-D-B and had only one resistor (the headlight dimmer) on the back. In both cases the light dimming resistor looks like a coil of wire (about 8-turns of special resistance wire) connected between two of the switch terminals on the back side. I don't know the resistor's specifications. Hope this will help.