AC Vacuum line replacement procedure

I am hoping someone who has done this can save me some trouble. My climate control is having the issue where the blower only turns on when in the defrost position and cold air shoots out the floor vents. No heat. I have found several posts saying that this is more than likely one of the two main vacuum lines that runs through the firewall up to the AC vacuum tank. In the engine compartment the lines look fine to the naked eye. What I don't know is how they look inside the cab. I have the dash fully removed, but I can't see how they come through the firewall. Is it literally just a long vacuum hose that runs through the firewall, or is there a plug on the other side to connect in to? I am afraid I will have to remove the heater core and AC unit to get to it which looks like quite the undertaking. Thanks for the help!