Does anyone know what or how the mr2 sdub controls the hot / cold setting, back from the heater control unit?  Is it controlled via a electronic valve or something different. 
Mine seems like it is not fully closing off the heating.. So air con (cold air) is mixing with some hot air thru the system.
Also does anyone know where this valve or what ever it is, is located?  Surly there is any easy way of testing it? I was thinking of clamping of the heater hoses to test.
 
If anyone can assist - would be greatly appreciated.
 

I think the temperature slider moves a cable (either directly or indirectly) which is connected to a valve in the frunk.

it is cable driven, they both are secure by a metal clip.
Cables goes to the front where opens and or closes the hot air. 
open the frunk and move the cable you can see it, is close to brake fluid resevoir 

If you have climate control I am pretty sure the volume slider moves a rheostat. Can't recall exactly, could be thinking of another car. But either way it ultimately operates the water tap in the frunk. You can take off the radio surround and you can see the mechanism.

Get someone to move the volume slider and make sure your tap moves to fully closed. Have engine running incase it's electronic.

Cheers