'89 Corolla strange A/C problem (heat mixing with AC)

Hi,
I have a mechanically excellent '89 Corolla whose AC has worked great
until these past 2 years. I believe the heat's mixing with the AC and
would appreciate your advice on how to fix it. The AC does function
and I don't think the issue's low freon.

Let's call the car's dash-level air vents #1 thru #4 to represents the
driver's side vent, 2 center vents, and passenger side vent. When I
turn on the AC, I get pretty cold air from #3 but slightly cold air
from the other vents. When the heat is off (I slide the temp dial all
the way to the left), I appear to be getting slightly heated air out of
vents #1,#2,#4.

I don't have a good understanding of where/how the heat and AC
ventilation systems interface and why I'm getting colder air out of one
vent than the other. How should I go about disassembling the
ventilation system (do I do it from the firewall or via taking apart
the dashboard? I assume #2) and what should I be looking for? I
assume that I should also take out the radio and the ventilation
controls and look around at how the ventilation controls are hooked up.
My mechanic's never been keen on taking apart the car's dashboard so
I'll need to do it myself.

Thanks for your advice,
John