Check out Brett's article on A/C upgrade to R134a:
http://www.koalamotorsport.com/article-airconditioning.asp

FloydR

Here's a link to the exploded view of your heator on the RealOem website.
http://www.realoem.com/bmw/partgrp.do?model=4073&mospid=47228&hg=64&fg=15

I have a Chilton's Manual that covers your car.  Here's what it says to
remove the heater assembly and core.

1)  Disconnect the battery ground.  Remove the instrument panel trim at the
bottom left.  Remove the package tray.

2)  Have the air conditioning system discharged.

3)  Remove the two bolts securing the trim panel underneath the evaporator
unit.

4)  Remove the tape type insulation.

5)  Using a backup wrench, disconnect the low and high pressure A/C lines
and cap the open ends.

6)  Unfasten the electrical connector for the evaporator.  Disconnect he
temperature sensor plug accessible from the outside of the evaporator
housing.

7)  Remove the two bolts and then remove the bracket that braces the
evaporator houing at the firewall.  Remove the mounting bolt from either
side of the houding.

8)  Unclip both fasteners and remove the housing.

9)  Now move into the engine compartment and remove the rubber insulator
from the cowl. (I think this is really a seal at the top of the firewall)

10)  Remove the mounting bolts for the cover which is located under the
windshield.

11)  Remove the mounting nuts for the heater housing located on either side
of the blower.

12)  Drain the cooling system then disconnect the 2 hoses at the core.

13)  Working inside the vehicle, remove the 3 electrical connectors for the
heater housing.  Pull off the 2 air ducts.

14)  Remove the 2 mounting nuts and remove the heater unit.

15)  Remove the air duct connections from the housing.  Push the retaining
bar back and thensplit and remove the 2 blower shells.

16)  Remove the retaining clips from the housing halves and split the
housing.  Then remove the core.

To Install:

17)  Reverse the removal procedure, noting the following points:
   a)  Cement a new rubber seal onto the core.
   b)  Make sure that when reassembling the halves of the housing, all the
distributor flap shafts pass through the holes in the housing.
   c)  Before reconnecting the refrigerant lines, coat the threads with
clean refrigerant oil.
   d)  Refill the cooling system with clean coolant and bleed it.
   e)  Have the air conditioning system evacuated and recharged using an
approved recovery/recycling machine.

They have another routine for just removing the core.

1)  Remove the center console and the glove compartment.

2)  Disconnect the coolant hoses from the heater.  Blow air into the upper
connection to purge the coolant from the core.

3)  Remove the screws from the holder.  Remove the screws from the right
holder and remove the holder.

4)  Remove the motor for the front vents and disconnect the inside
temperature sensor.

5)  Disconnec the screws, straps, and clips holding the heater cover.
Remove the cover to expose the internal coolant pipes.

6)  Remove the mounting screws from the coolant pipes and lift out.  The
heater core removes from the right.

These don't make much sense to me but hopefully will be more helpful to
someone looking at the hardware.

P.S.
The trick to removing the clips without breaking the housing lip is to use a
small pin (like a #40 drill) inserted into the hole in the side of the clip
to use as a prybar to open the clip while lifting it off.