I just looked into mine tonight also and had the same thing happening.Heres what i found.First the vac line tees into the one the that go's to the fuel pressure regulator near the firewall,then go's into the car and from what i can tell it go's to vac switch on the heater control on the left side.That vac assembly controls the valve that is on the heater hose near the A/C box in the engine compartment.It sends vac to the valve to shut off the the flow to the heater core.Mine was leaking inside at the heater control switch and i had to make a piece to make it seal better.After fixing that the valve was working again but i still only got heat out of it.So i started tracing the hot,cold lever cable to the heater box under the dash and found that the swiveling lever broke off of the top of the box.If i pushed it towards the firewall it would blow cold air.Its easy to get to to check,just swing the glove box down and its right there.You just have to look up alittle bit.Im still not sure how im going top fix it but tommarows another day,lol
