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Right now I have no control over the heating system. PO stated that it worked great, assuming he was tellilng the truth and it wasn't sales BS, is it likely that he disconnected the vacuum source or one of the other control tubes when he installed the aftermarket radio? How protected are the vacuum lines behind the dash?
About all you can do is try to track down the issue. The hoses are not really protected but they can be difficult to remove. The chart below is from a 1972 but it is the same as your car.
Are you sure it is a vacuum issue? You can tell real quick by reading the manifold vacuum first, and then pinch off the black supply hose to the heater control. If the vacuum increase's on the gauge when you pinch off the black hose then yes you have an issue inside the car.
Willcox
Last edited by Willcox Corvette; Apr 12, 2012 at 04:24 PM.