http://tinyurl.com/2awat2

http://www.docrebuild.com/dr-r-web/8860203.HTML
http://www.docrebuild.com/dr-r-web/8846401.HTML

As best I can tell, according to Chilton's, it's not a simple job...

If you follow the heater hoses from the engine (on the passenger side)
you'll see that they fit onto
a pair of nipples on the firewall, one above the other. Those nipples
are part of the heater core,
which is contained in a plastic duct under the passenger dash. The
nipples are near the inside
edge of the heater core, so the heater core is mostly outboard (towards
the fender) from them,
but on the other side of the firewall, inside the car. The heater core
is rectangular (perhaps about 12x18)
and lies flat against the inside of the firewall, and the inside and
outside air ducts sandwich the firewall
between them, enclosing the heater core. I believe the inside duct has
studs fixed on it that extend through
the firewall and through the outside distribution duct, and those nuts
sandwich the firewall and a pair
of sealing gaskets between the two ducts..

On the firewall in the engine compartment you will see a molded plastic
cover (distribution duct) held
onto the firewall by nuts on studs... it is rectangular, with a round
section on the extreme upper left as you
look at the firewall from the engine compartment. There is a raised
section of the duct cover that
starts on the rectangle and ramps up/out towards the round section; that
round section houses the
heater blower motor (which sticks out another 3 or 4 inches); the blower
motor may actually
be beneath the top of the front fender and is bolted to the distribution
duct.

The rectangualr portion of the cover/duct is roughly the size of the
heater core behind it, but
unfortunately the heater core doesn't fit through the hole in the
firewall, it has to come out into the
passenger footwell inside the car, which requires a bunch of other
ductwork stuff be removed first.

Here are the instructions from a Chilton's shop manual for a NON-a/c car:

1. Disconnect the battery ground cable
2. Drain the cooling system and remove the water hoses from the heater
nipples

3. Remove the air-distribution duct stud nuts on the firewall (about 8
nuts, I think) and move the cover
    outward off the studs
   (then you may want to plug the heater core nipples to avoid leaks
when you remove the core)
4.  Remove the right instrument panel pad, right side dash braces,
center dash console duct,
     and the floor outlet duct.
5. Remove the radio and center dash console
6. Pull the distribution duct off the firewall from the inside... remove
the resistor wires when clearance is sufficient
   (I think the resistor wire connects atop the distribution duct, 6 or
8 inches inboard of the heater core)
7. remove the heater core retaining springs, then remove the core
8. reverse the above to re-install

A while back in this thread, you mentioned that the temperature is low on
the temp gauge. If you don't have heat in the engine, you won't get heat to
the heater. Low engine temperature indicates that the thermostat is stuck
open. It's a cheap and easy replacement - do this first before you start
tearing apart the rest of the car. And get a friend who knows something
about cars who can help you.