Ulf, your were right : the correct connector is the box closest to the driver on the lower side of the bracket.
So, the last connector I opened was the good one.
On my car, (US 71 280SL), it is a Bakelite box, and the cover is fitted with 3 screws (instead of a "clip" cover on the other two connectors).
My dash lights are now working well, except for the upper right corner of the central cluster (I suppose a bad bulb), and the clock.
Is the clock light commanded by the rheostat, or is it independent ?