Hello,
Final update.
It is OK now!
A wire broken somewhere giving from time to time an "half open circuit".
After having checked all the wiring without result the garage has decided to run a new wire from sensor to the compass unit (ground was OK).
They charge me only two hours !
They have really passed one day and a half to check everything including BCC and ACC control panel.
In fact on the 2000 Blazer there are
2 air ambiant sensors (one on hatch support, one on radiator side behind the grille) the one for the overhead is on hatch support the other is for AC and the schemes from GM are splitted and do'nt specify the 2 sensor but only the indication outside temp sensor.
So it's easy to understand that there is only one sensor and to search on the wrong way ........
The overhead console sensor was falled behind the hatch support under leaves at bottom of the radiator which was confirming the possibility of only one sensor..... Wires were hurted due to that......
Many worries for nothing but happy to know that it's not frezzing in France mid of July!!!!!!
Once more thanks for the support of a very useful forum!