Wiley Coyote

Hello everyone, I have an 02 Blazer that I'm trying to bring back to life. The issue today is 4WD, specifically the vacuum actuator for the front differential. Where does the line connect opposite the the Vacuum switch? I've traced the line from the switch, under the battery tray, over the right valve cover - back under the distributor then down along the Trans dip tube. from there ??? Could someone point me in the right direction?

The problem is the front axle won't engage. I'll probably replace the Vacuum switch but I want to check the vac Line back to a source to prove the fault.

Thanks in advance for any help.