The tailgate wiper doesn't go to its park position and the tailgate window is not opening.
The result; I'm not able to open the tailgate anymore.
At first I checked the relay for the window and the wiper; they are ok. The next step is checking everything inside the tailgate but the tailgate is closed .
I know that the CCU is not allowing the the tailgate window to move when the wiper is not parked.
My question: How can I open the tailgate without any damage to the window?

LRRON, then the wiper is turned off, does it stop where you turned it off, or does it continue for a while and end up anywhere?
If it stops where you turned off the power, it may be worth while having someone turn off the power as you are applying a little pressure to the arm to return it manually into the parked position. if it won't budge, then don't go any further with the pushing it back..
on some of these motors, (inside the gear mech.) there is a metal band which creates a circuit when the arm is in the parked position - thus allowing electrical connectivity to energise the opening switch / mechanism.
(only a thought.... - not had the problem myself.... but if you DO have to move it manually.. it could mean the motor is on it's way out.)

xpuser8334, the wiper is not stopping when I turn it off and moves to its normal stop position just above the rubber where it should be.
A few days ago I saw the wiper moving from that position to its parking position on the rubber by itsself. It's not doing that anymore now.
According to the manual the wiper should go to its park position when the tailgate window gets a demand to move downwards.
I suspect a faulty wire but how do I get into the tailgate?

It could be the micro switch in the door handle rather than a wiper fault. Mine doesn't go the park position but the door opens ok.

Thanks for the response, it's possible that it is the micro switch in the door handle but how do I get into the tailgate?

Climb into the back and undo the tailgate door panel... a row of screws are at the bottom of the panel. with the panel removed you can undo the door handle bolts to check the micro switch
Hope this helps