There is vacuum leak.  The climate control system sends hot air to
windshield when there is no vacuum to drive the door elements.  This is
the defrost mode for safety.

You need vacuum gauge and pump to continue debugging.

At the yellow check valve (near fuse box), is there vaccum in the line
(coming from engine side)?  If there is no vacuum from engine side,
check vacuum lines there.  At this age, the vacuum line in the engine
bay should be pretty bad if they are the original.  On the other side,
the interior vacuum lines should not come loose or break unless it is
disturbed.  Was there any work done behind the console or under the
dash?  If so, check there.