Defrost IS the diagnostic. If it doesn't blow lots of hot air then, it's
busted.

It's full heat on, full blower un, upward windscreen vents open.

I'm tempted to say monovalve:

    http://articles.mbz.org/hvac/plumbing/monovalve/

But because you can get both hot air and cold air, maybe not, usually (USUALLY(
it failed stuck open or closed but seldom varying. But it is possible. The mono
valve controls the temperature of the water reaching the hater core by being
pulsw width modulated by a controller near the blower which is told what to
do by the pushbutton assembly.

At this age you probabky want to replace the monovalve anyway - the insert is
cheap and that rubber bit is about worn out anyway.

If that doesn't fix it (it might) then find a good used pushbutton assembly (ebay?)
as 99% of the problems with heat in these chassis is that wretched thing. I've still
never seen any car that had any other problem with any other bit of this one of the worlds
most arcane and overcomplicated heating systems. Keep in mind the manual for the HVAC
is larger than the manaul for the entire engine/chassis.