Folks,
My 1998 1500 2wd only has air out of the defrost vents. Troubleshooting found there is no vacuum going to the controller (believe supply should be the black line). Where does this line go? I found a broken vacuum line to the under-cowl vacuum reservoir and fixed that, still no vacuum to the hvac controller.

 

The line goes through the firewall next to the a/c lines (to the right)… usually chaffs through and breaks at the firewall… if you’re lucky and can retrieve an inch or so back through the hole, you can patch it together with some 1/8” rubber vacuum line and some silicone.
If not you have to remove the dash… at that point you may as well replace the whole line.
I opened up a hole between the two pipes so the line routing kept it away from the firewall so it wouldn’t chaff in the future.

 

I myself have been dealing with this as well. I did get a hold the line you spoke but it still didn't fix my issue. Mine a 99 and I have what looks to be two line by the T's in the line. Once I get other top issues fix and I save up for the top part of the dash. I am just going to drop the dash and run through everything. I also notice a opening behind the motor that looks like a line may have entered there as well.

 

You may want to look through the HVAC sticky here.