Defroster circuit closing

[Michel]

I took the truck for a short journey on Saturday morning and after few kilometers found that the temperature was increasing in the cabin even I was driving with windows opened. I check the defroster and found it hot as there are no valve to close the coolant circuit.
I plan now to install a valve/tap in order to close the circuit but don't know on which hose to install it:
The one coming from the water pump or the one on the thermostat housing?
Is there no risk for the water pump to install a valve in the circuit.
Any idea ?

Thank you
Michel