As you say there must be a subtle difference in the outputs of the aftermarket panel that messes with the V8 HVAC signals to the heating valve. There is a HVAC ECU on V8's that I can't find listed as a standard 75/ZT item. Unfortunately as Rimmers no longer provide part numbers on their listings for NLA items it is difficult to do a conclusive search, but I cannot see the same item on the 75/ZT heater assemby diagrams (Item 5 on the V8 diagram).
I have often thought that gremlins in this additional ecu is what causes heating problems on some cars and not others, or possibly just a dodgy connection to it. The HVAC has always worked perfectly on #431 (touching wood as I type!) but fitting the aftermarket panel showed the ecu was sensitive to different inputs
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