The light switch grounds to the cab's dash to complete the circuit, sometimes they can be very tempermental.
To really pin point the problem, I'd use either one of the screw drivers that lights up when finding a "hot" wire, or digital volt meter. These 2 tools can be you best friend when hunting wire gremlins (ask me how I know LOL).
Twist the light switchknob after pulling on the knob to the 1st or to the 2nd click. Still nothing, make sure the 2 harness plugs are plugged into the main wireharness under the dash.
This plug is located on the right side of the steering column straight up, and bolts to the cab firewall way up under the dash.
Also check the wires in the wire harness to light switch. Check for "damaged" wires that got hot, corroded, or even broke. If there is, this is probably the source.