What you have is a plugged AC evaporater.  The air conditioning has a core
that looks like the heater core, same basic idea.  They are known to plug
up.  When I first encountered the problem on my 92 cadillac a repair shop
showed me an evaporater that they had removed and it was plugged solid with
some kind of thick gooey film.  He told me that it was caused by a leak in
the evaporater which allowed the oil to ooze onto the outside fins.  He
claimed that dust then stuck to the oil and plugged it up.  His storey
didn't sit well with me, a leak would have had the oil centralized around
the hole, and this was evenly covered. I still had good cooling just no fan
so the leak thing just didn't wash. I did some further research and
discovered that the film is actually mold growing on the evaporater.  This
made a lot more sense.  Moisture collects on the evaporater when it's
cooling.  If it doesn't ever dry mold will grow.
You need to get at the evaporater.  Yo can usually clean it without removing
it.  You need to remove an access of some kind from the engine side of the
firewall.  Often removing the blower motor will provide access.  I think GM
sells some kind of cleaner.  Essentially you spray this stuff on and  blow
it out with an air hose or water hose with good pressure.
Steve

motor (just changed) to high speed but hardly any air blows through
vents/floor or defrost into the car. The air trickles out, its hot, but just
no volume and we can't get warm. Any other 'things' I should be looking at?