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The quick heat system apparently is not an electric element at all. The system in fact when enabled places a restriction on the exhaust and thus creates back pressure and accelerated. This is very similar looking to an exhaust break causing simulat misconecptions about an exhaust brake being fitted.
This is a vacume operated diaphram controlling a gate valve below the turbo.


While I believe this to be the case with the earlier (3.1 litre) engine, this is apparently not so with the 4JX1 engine. The official Isuzu workshop manual wiring diagram for my 4JX1 clearly shows wires attached to this switch and leading to an electric element.