No heat on passenger side

I have a 1999 Grand Cherokee limited edition, The drivers side vents work great one heat and AC, The passenger side vents only blows out Cool air. Nothing has been worked on under the dash, everything worked great up till a month ago. I can't figure it out. Any ideas???

I would check the vacuum line to heater control. Start at intake make sure the fitting isn't plugged. If broken try putting a slightly piece of rubber hose about two inches long over the two broken pieces.
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