(following of previous message)
He also replaced the O_ring of the heater control valve, which was totally stuck (not easy to reach).
It was a long and difficult task to put the heater control unit back in place, but I learn a lot, and it will be easier next time, with experience.
The tech manual was very helpful. Without it, and its very useful information and pics, I would not have started this process.
Now I have a "new" dashboard with everything working again : speedo, heater unit and clock (revised by JDO instruments in UK).
Next projects : fix a rattling window, and by pass the rheostat, operations which are also very well described in the tech manual.