I know this has been raised a couple of times already, but from all that I've read, it still seems that the problem hasn't been satisfactorily resolved.
I have a 2009 SR Hilux....no climate control.
I found out a couple of years ago that the clicking sound that was annoying me came from 2 relay units located behind the glove box (at about 1.00 o'clock). The clicking noise only occurred when I turned on the heater in winter while the engine was still cold. Once it warmed up, the clicking stopped.
These are relays for the Positive Temperature Control elements in the heater box that provide warm air to the cab until the engine water temperature is hot enough to take over from the aluminium heating elements.
Boxhead might correct me if this is not the case.
This past week, however, with the outside temperature in the high 30's/low 40's celsius, the relays have been clicking for the first couple of minutes after start-up.
This has never happened before.
I'm happy with the time it takes for the car to reach operating temperature....even in winter....so, for me, the electric heating elements are not really necessary. So, today, I removed the 2 relays that provide the power to the elements. All seems fine.
I can't see if this will affect any other circuit. Perhaps Boxhead can shed some light on this as well.
Hope this helps others.