Either you can have a leak from the internal venting system that is causing this or you have a leaking heater core instead.............
because of condensation
Sorry bad news Your heater core has a leak
Probably a vaccum leak. The vents default to the defrost vents (Windshield) when there is no vaccum. The main vaccum connection is on the throttle body. The thin lines become brittle and break very easily. Good luck finding the leak.
you should probably run a duster thorgh it and/or check the pipes to it with it running =there might be an air leak
It will if the condensation drain is plugged.
Water leak.
Check the floor heat vent and defrost to see if air comes from them. The vents will be vacuum controlled and a vacuum leak will make the vent doors divert air to the wrong place. Check the entire engine and climate control systems for a vacuum leak.
Dont know what the problem is but i can tell you its a vacuum leak . the vents are vacuum operated and require air pressure to change between the vents and floor. the defroster is default setting you may have a vacuum line that has come undone but all i know is its somewhere under the dash.
Very poor installation
Air vents in most cars are vacuum controlled. Check for vacuum leaks in any part of the engine system. Also check the ac/heat control system, as they can develop a vacuum leak with time.
Check the 96 Blazer for any vacuum leaks in the system. A vacuum leak can cause the air flow to only come out of the heat vent.
Check to see if the coolant level is low. If you have a leak in the system and the level drops low enough your heater can't do it's thing.