W124 230TE heater problem

Hi guys,

I am in Australia and I have a 1986 230TE, it has the standard dual zone with air conditioning. My car is right hand drive.

I have search this forum and others but I can't find any info relating to my problem. I am hoping someone else has experienced this issue and can tell me how it was fixed.

When I move the temp wheel from blue to red on the LEFT side, I get regulated hot air at the LEFT side vent, and when I move it back to blue the air goes cool again.
When I move the temp wheel from blue to red on the RIGHT side, I get regulated hot air at the RIGHT side vent and when I move to back to blue the air goes cool again.

But, the problem is that when I move the RIGHT side temp wheel to red and get the regulated hot air at the RIGHT vent I also get full hot air at the LEFT side vent, irrespective of where the LEFT temp wheel is set.

This full hot air from the LEFT vent eventually heats up the cabin to the point that the cabin sensor detects that the cabin is too hot so then the car cools the air on the RIGHT side as it is supposed to do, so I end up with hot air from the left vent and cool air from the right vent even though the left temp wheel is blue and right temp wheel is red.

I have changed the push button / temp wheel unit in the centre console with two other spare ones, and I have changed the temperature regulator box that sits under the glove box with another one. I have also taken the top half of a duovalve unit and sat it upside down in the engine bay plugged in so I can watch the little pistons pop up and down from the driver's seat. I can see both the pistons moving up and down when I only move the right temp wheel.

So, if the problem isn't caused by the pushbutton unit, and not the temp controller unit and not the duovalve, then what else could it be?