help finding positive headlight source in caf

[CPucci41]

Hi y’all, I’m installing a glow shift PSI gauge in my cab for my onboard air system. I’m goond with switched power and ground. Need some help with  headlight source to dim when the headlights come on.

 

instead of sitting out there with my multimeter for a few hours was wondering if anyone knows where I could pick up a positive headlight source in the cab?

 

Glowshift recommends not using the dimmer as the led would flicker. 

thanks