Headlight dimmer switch 2002 P32

My headlight dimmer switch will not switch the headlights to dim.

Is the dimmer function controlled by the multi function turn signal stalk (see below) or something else?

https://www.workhorseparts.com/2002-P32-Chassis/25111446-Turn-Signal-Lever-(Headlamp,-Cruise,-Windshield-Wiper-Washer)?filter=283

Thanks for your help.