You shouldn't hear any hissing from under the dash, not an air like hissing anyway.
Both the 5.2 and 5.9 run their accessory vacuum systems off the back of the passenger side of the block. You are going to be looking at a bunch of thin plastic tubes. Depending on whether or not you have 4x4 will determine if there is an unused port on the vacuum splitter, if no 4x4 it should be closed off and not pass any air.
When my vent control stopped working it was becuase one of the rubber boots for the vacuum line had degraded over time and just came apart.
A really easy check to see if vacuum is making it to the vent control switch is to change the vent position with the engine off just after running it. If you hear a hissing, then there is vacuum getting to that switch but not the doors. It there is no hissing, i'd go straight to the vacuum hub.
Hemi395 is right, the vacuum lines enter the cab right behind the engine through the firewall, they are pretty easy to spot.