Unfortunately the heater box was beyond economical repair. The entire bottom section was paper thin and had rusted through in places. The side joints hadn’t fared much better and had rusted from within causing the joints to swell.
Every time the heater housing was rotated to inspect it, showers of rust fell from every opening! The motor and fan cage were also missing.
The heater matrix within is surrounded by a thick felt material and I suspect that this had acted as a sponge. The absorbed water had sat against the base and sides causing them to corrode over a prolonged period of time.
Fortunately new heater units are available (although not that cheaply), so a new one was ordered along with a heater matrix and a kit containing all the various padding materials.
Oddly the padding kit didn’t include the square rubber seal fitted between the heater body and the bulkhead.
If you were going to the effort of replacing all the internal padding, it’s likely you’d be working on the fan housing off the car and so would probably need to replace the bulkhead seal too!
On the positive side, the new heater has an improved design for the fan cage which hopefully might address the reported problems with the original, asthmatic unit. The blades on the original fan cage were flat and aligned radially which isn’t the most efficient in generating a throughput of air. The new cage has curved blades angled towards the direction of rotation.
The heater motor can be switched between two operating speeds and is achieved by introducing a resistor into the circuit to reduce the voltage across the motor. It’s riveted to the motor’s mounting flange and the loom wires soldered in place rather than using spade connectors. I’d stupidly expected a new unit would come with the resistor attached!
The padding around the heater matrix is a thick fibrous material and it was a really tight squeeze to fit it all in. At least the matrix won’t be able to move around!
I found it was necessary to glue the square foam seal to the heater box with contact adhesive, before fitting the heat to the bulkhead. Otherwise, with only one pair of hands, it tends to fall out of place when attempting the fiddly task of fitting the mounting bolts while supporting the heater unit.
| Heater Matrix & padding | Bulkhead seal glued in place | Heater unit installed |
|---|---|---|
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Fortunately the rubber connectors and ducting behind the dash were all present and in good order. So they only needed cleaning in soapy water to remove the grime that have built up over the years.
To the right are diagrams detailing the external terminals and internal wiring for the four 6RA relays used in the S2.
With the exception of the 3-pin cooling fan relay, the W1 and W2 terminals are used to energise a coil winding which has an iron core at its centre. 
The 3-pin relay lacks a W2 terminal because it is designed for applications where terminal C2 is always connected to 12 volts. Internally the C2 terminal is also connected to the coil winding and so acts as the W2 terminal as well, delivering 12 volts. The coil winding is therefore energised by grounding terminal W1, resulting in the switching of the relay. 
The switch can be split in half by gently prising the retaining tabs outwards. The tabs are made of pot metal so I wasn’t sure they would survive the operation.