when u turn the heater dial, it moves a cable similar to a brake cable. the cable then turns the diverter mechanism which controls how much air flows through either the heater matrix (hot) or straight to the vents (cold). a valve in the heater matrix controlling the flow of hot coolant is connected to the mechanism via a rod.
so turning the dial towards hot pulls the cable, which diverts air through the matrix core and opens the coolant valve.
you should be able to turn the dial from fully cold to fully hot without it stopping prematurely. if it stops short it needs adjusting.
the cable is adjusted by where the position of the outer cable is locked on the control panel with an e-clip.
i think u can access the back of the control unit by removing the central console cover, remove the radio and u'll be able to peer down at the control cables.
i adjust the cable tension by removing the e-clip that secures the outer cable to the unit, turn and hold the dial at full hot, pull the outer cable outwards so when the dial is at full hot the mechanism is at full hot, then secure its position with the e-clip
