L400 heater heat control question

[frostyfriday]

Hi all, I have a 95 Delica Exceed L400 SWB.

The last few years my heater has been a bit unreliable. It is capable of heating really hot but it seems that sometimes the temperature gets stuck. Sometimes cold, sometimes hot. Right now cold when it is freeing!

I have the heater/ac controller MB946619 that has the old vacuum style display.
I am an ex electrical engineer and understand the feedback systems required to run a system like this. It feels like it is not a feedback problem but rather a stuck valve that controls the hot water flow. I am assuming there is a small servo type motor that drives a flow control valve of sorts that allows more or less hot water from the engine into the heater system.

Does anyone know where this valve is situated? I already have half the dash apart in order to fix the heaters display board so it is probably a good time to chase after this problem too.