The Heater

The heater works on the principle of a live connection to the motor (with the ignition on) and everything controlled by the earth. Inside the heater matrix there is a 3Ω resistor (it's there as it may become warm) and this is used to give the slow speed. The 'earth' side of the motor goes to the heater control on the dashboard and is controlled by the 'Pull Boost' knob. When the knob is fully in, there is no connection to earth and the motor is off. Pull the knob to the first position and the connection to earth is through the 3Ω resistor giving the slow speed. Pull the knob fully out and the resistor is bypassed and this gives the fast speed. The circuit appears below:

The 'Pull-Boost' switch connects the earth to either (H) - the High/Fast or Boost speed, or (L) - the Low/Slow speed, when the current flows through the resistor. The cables on my particular GT6 are as follows:

Green From Fuse to Heater Motor (+ve)
Slate Heater Motor (-ve) to Resistor in Matrix
Green/Yellow Resistor in Matrix to Pull/Boost control (Low speed) - passing through resistor
Green/Slate Resistor in Matrix to Pull/Boost control (High speed) - bypassing resistor

In the official Workshop Manual circuit diagram for the MK2 GT6 the Green/Slate cable has no colour specified, and the live wire to the motor is white, not green! 

The 3Ω resistor can be seen below:

The Heater Controls

The heater is controlled by two knobs in the facia, above. The right knob operates a valve on the rear of the engine via a cable. The left knob labelled "Pull  Boost" controls the direction of the air flow, and also the motor speed. Looking inside, this device is quite clever in how it controls two functions with the same knob, see below:


The position of the lever controls the direction of the flow by a short cable (not connected, see below) to the heater matrix flap.  The curved contacts at the top ensure that contact is made irrespective of the position of the lever - at present in the "CAR" position.