Here are ALL of the possible causes...
As far as the HVAC...
1)Defrost is default.
a)When the vacuum source to the HVAC is lost the system defaults to defrost for safety reasons.
As already stated you need to check the vacuum source into the passenger compartment from underhood.
2)Cold air is going to be caused by...
a)Air in system preventing coolant flow through the heater core
b)Faulty heater control valve
c)Restricted heater core
d)Temperature control cable not connected
e)Blend door broken
As far as the Overheating...
1)Head is not sealed correctly
a)Head is warped
b)Head is/was not torqued properly or stretched headbolts were re-used.
2)Faulty Water Pump
5)Collapsed Radiator Hose
6)Air In System
7)Water Jacket Restricted
8)Cooling Fan Inop
9)Front Lower Air Deflector Missing
HVAC VACUUM SOURCE
(Your car will be similar, may not be exact)
The rubber hose runs to a small polyethylene hose that runs through the firewall to the HVAC control unit.
PROPER DRAIN AND FILL PROCEDURE