Gmc truck hose on manifold

There is a hose on top of manifold and it goes to the back of dash board what is the name of the part the hose is connected to by the back of dashboard

Answer: Does this hose actually go inside the truck and into the dash board, or does it go the the firewall under the hood? Is it a small hose about the diameter of a pencil or is it quite a bit larger?

If it is a larger hose that goes to the firewall, then this is a heater hose going to the heater code. There are actually 2 hoses that connect to the pipes of the heater core.

If it is a smaller hose then this is most likely a vacuum hose that goes into the dash and controls the temperature and vent door positions as you turn the knobs on the a/c heater control panel.

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