Hi Colin, removed the heater housing from my Flavia fairly recently. Held in place with four bolts from memory 6mm, the ones at the front fairly easy to get to but those at the rear against the bulkhead a little more difficult . I had the seats out at the time which allows you to lie in the footwell and work upside down. Bolts at the bulkhead had corroded following a leaky unit but can be accessed from the engine bay and penetrating oil applied, you will find the end of one just inside the metal cowling that the flexible inlet pipe attaches to( within the engine bay ) and the other in the opposite location near the wiper mechanism, can also be drilled out if needed. Replaced mine with stainless bolts. Unit drops out once all other connections disconnected and then remove the top panel of the unit to reveal the matrix and large flap which controls airflow.
The flap mechanism on mine had seized but easily repaired and then i replaced the matrix with one i bought online which was almost the same size, although the outlets were in different locations i managed to route the water pipes to suit. Tricky getting it back in with the foam gasket in the right position so glued in place before refitting. I think i also removed the clutch pedal to allow access to the cable fitting which is rather awkward. Good luck, regards Nigel.
