Qestion is about my defrost. It does not blow. I close the vents push defrost button and get basicly no air. It is however blowing out of the floor still. If I turn the defrost button off and the floor on the air flow on the floor is still the same as when defrost is on. Is there a simple repair for this. Vaccum lines??? I also have excessive fogging on all windows could this be a bad heater core. If so how hard to replace? Please help.

Definitely a problem with the little vacuum lines. There is one that comes in through the firewall from the manifold. It travels to a vacuum amplifier (usually looks like a set of white plastic balls.) From the other side of the amplifier, it enters a common rail on the backside of the FLOOR/DEF/REC buttons. A hose then splits off for each function. The floor hose then tees into two hoses which actuate the floor flaps on either side of the bottom of the fan housing (inside the housing.) The REC hose runs up to the cowl intake. The DEF hose tees and runs to valves on both sides, mid-dash, which actuate the defrost flappers in the air duct.

Make sure all these hoses run where they're supposed to and that they aren't leaking or torn (also the valve bellows). It is possible you have a bad button (leaking vacuum), but that would most likely make a sucking sound.

I had this problem a couple months ago...

Very simple fix(and you have plenty room to do it:thank you volvo!!): check the tiny vacuum line starting at the intake manifold, going to the firewall, middle-right side. For me, the hose was toasted and there was many, many little cracks on it so, the vacuum was not going to the heater system.

I also heard problems caused by this tricky one way valve between the vacuum tube and the manifold.

to see where it goes indoor, I think removing the glovebox can show it.

good luck