Thank you all for your comments ... although it is very hard to understand all of your thoughts.
I didn't know about the front dome light was preset the circuit for the two lamps in the "normally closed" position (the original GM schematics seems to show two SPST switches), so I designed two alternatives solutions with diodes or a relay. Then, when I removed the dome light, I checked the circuit and I found the "normally closed" position already prepared for open-door control (two SPDT switches). In this case the solution is very simple.
The schematics to use is the last posted in the italian forum.
You only have to pass two new cables through the internal car roof, from one dome light to the other.
It could be do with a single cable, but, in this case, if you switch the center dome-light command in the "light off" position, also the front light will always off, as if you switch the control in the "manual on" position, the front light will always on.
Again: the circuit is trivial and it's not necessary an "international brain-storming"...
Anyway, if someone has some problem, i can upload the schematics with original and modified circuits with connector's pin number and cables colours.
Dimenticavo: un saluto particolare a n1k0, e complimenti per il suo italiano!