Now, that I have done the retrofit I can report to you how it worked:
My car was prewired. So everything was Plug & Play.
In the trunk I only had to connect the 3 wires. No problem. Watch out, that you buy the right nuts for the bracket. My dealer gave me wrong ones.
Installing the BT antenna is also very simple. Open the three screws under the steering wheel and open the compartment. Now you can clip in the antenna and connect it to the wire.
Installing the mic is a bit more difficult, because you have to remove the roof control center. I used a very small screwdriver to clip it out. But there are several ways described in the forum. Removing the blind plate is a bit difficult but with two very small screwdrifers you can clip it out and lift it.
Now the most difficult part: The rear backrest...
First remove the backseats. You just have to pull it up and it clips out.
Under the armrest there are two big T20 or T25 screws. You need a big tool or much armpower to remove them. But be carefull!!!
Then you have to go into the trunk. There are two holes, the emergence exit
. Put in a screwdriver and turn the little plate till the backrest jumps out.
Now you have to take it out. Don't try to pull it towards the front seats and work with the backrest still in the car.
On the right side there are the Most-Bus connectors. Just follow the green cables. Remove them and try to clip the covers out. They are very hard to seperate. I broke both of them so better buy two new ones at your dealer...
Now you just have to disconnect one wire of each connector and put it into the other connector. Use a small screwdriver for that.
Now why you had to remove the backrest completely: Clean the holes for the screws. They are fitted into the holes with some sort of glue. You have to remove it completely.
Then put in the backrest and clip it into the "emergency exit". Now put in the screws. But be careful! I destroyed one of the threads...the screws are very hard and the threads are very soft. But it also works with one screw in place
Now you have to go to your dealer and reprogram the retrofit.