Interior heating temperature drops on one side

[VosCoBoss]

Hi,

I have an Avensis 2.0 D4 VVT-I (petrol). Equipped with climate control with seperated left/right temperature control

For the first 20 minutes after a cold start the interior heating works fine. After 20 minutes of driving the temperature on the driver's side (which in my case is the left side) drops. If I increase the temp for the left side by 1,5-2 degrees, it matches the right side again. However, the footwell temperature on the same side never drops. The car has been back to the shop where they changed the thermostat and reset the heater flaps. Didn't make a difference. For further diagnostics they would have to remove the entire dashboard which would be very costly. I could now start to replace components one by one and hope to solve it that way, but I don't know where to start. I hope someone can point me in the right direction...