Looking for 4wD vacuum actuator location

Hey guys
Have a 91 super custom and my 4wD doesn’t seem to be engaging and no dash light when button pressed.
Was told to check if there’s suction on the vacuum actuator hoses but am not sure where to find them or what they look like in order to do that.

Would super appreciate any thoughts or suggestions on this or how to do that. (Checked fuses already) No dude so will take all the help I can get lol!

Happy Hiacing
Carley