Fabia Tailgate/Door Sensor Replacement

Hello everyone,
My brother came to me yesterday with a strange phenomenon he was experiencing with his Fabia, manufactured in August 2005:
The interior light is not turning on, and neither is the warning buzzer. Therefore, it seems logical that the central locking system automatically closes after about 30 seconds because it doesn't know that the door has been opened. It's almost certainly due to the door contact sensor or the wiring harness.
The door is opened and closed approximately 80-100 times per day (for mail delivery).
However, during the ride, it occasionally lights up for a few seconds.
The funny thing is: in 9 out of 10 cases, my brother has to open and close one of the rear doors briefly before the tailgate will open.
According to his statement, both events occurred simultaneously.
Is there a more or less logical connection between the front left door sensor and the tailgate?
So, the biggest problem is actually that I don't have a circuit diagram.
What's the name of that funny software from VW that's used for planning?
If anyone can help me, please contact me.

HAPPY EASTER AND LOTS OF COLORFUL EGGS!

With so much opening and closing of the door, there's likely a broken cable between the door and the body of the car! Simply remove the rubber boot and inspect the cable! The same applies to the tailgate!