I disconnected the battery before I began (black pole first, then the red one). I fitted the rings, and I disconnected the 2 cables of the instrument cluster. And this is something you shouldn't do. The transponder/immobilzer is located here, and if you disconnect this, your car won't start anymore
The anti-theft signal comes on when you try to start the car, the engine runs for a second and then it dies. (You're still able to open the car doors).
My car (9N3) is now in the garage, hopefully I don't have to pay much
-update-
Just got my car back. 50eur for deblocking the transponder system en recoding the two keys ...