Hi everyone !
Some news about this little retrofit. I found a 98' steering wheel last week, the thing arrived two days ago, I already cleaned refurbished it with ColorLock repair kit

But the interesting part is to come. I removed the original steering wheel from my car, to verify the GM code on the rotative connector or "SRS ring". The one on the car is 90507512 : it's indeed the one with 4 wires (2 yellow and yellow/white for the airbag, and 2 brown and brown white for the horn's buttons)

Then made some research about the one wired for radio control, it's 90507514 which has 6 wires.
BUT

The car's loom that's plugged at the back of my 90507512 SRS ring has 6 wires. And all those wires colors match with the later model Omega.
You can see :
pin 1 = Brown (ground)
pin 2 = Brown/White (horn)
pin 3 = Red/White (radio control+ ?)
pin 4 = Blue/Red (radio control- ?)
pin 7 = Yellow/White (airbag to unit control)
pin 8 = Yellow (airbag to unit control)

It's as described on the Later Models diagram

And not corresponding the Early Models diagram, as there are only 4 wires coming out of the U20 (U20 is the SRS ring on both diagrams):

Now I must check where the two Red/White and Blue/Red go, should be at the back of the HU but pined to something non attributed on the radio while this HU is not compatible.
Opel already had pre wired car loom for this feature maybe from a certain year and month.