4WD dash indicator light on '77 with TH350

[mw67rs]

I have recently acquired a '77 Chevy truck. It needs a new engine so I have purchased a crate engine for it. I was driving the truck to test out various systems to see what needs attention. This is my first 4wd truck so I don't have much experience in the 4wd system. My truck has full time 4wd. When I shift the transfer case into Lock the dash indicator light comes on some of the time but not every time. I think the lock is engaging but cannot say for sure.

Does anyone know how this indicator light is triggered? Is there a switch in the transfer case or linkage somewhere that triggers this light? If so, where could I find it?

Also, any way to tell if the Lock is engaged even if the light is not illuminated?

Thanks for any advice.