The heater box splits into two pieces. The plate (1) mounts the HOT/COLD cable, and is held by three screws. The mixer door is held in by and pivots on a pin (2) which must be removed.
Note the position of the flat "tang" on the end, it only goes back in one way.
The pin in the fourth picture goes through the end of the "blend door actuator".
The circled screws in photo five are under a foam seal which must be removed to access them. Keep it to put back on. The rest of the screws are easy enough to fin, remove them all.
The last pictures shows the halves being separated.
