my german is a bit rusty but if i read the wiring diagram right on the central control panel behind the seat relay 4 is the reversing relay , Fuse 21.3 (4F1) 10A fuse is reversing fuse and also there should be a 6pin connector 4X2 that should have the reversing signal where you can tap.
check it with a meter and see if i'm correct.
you can also look around the rear of the truck in the counterweight or along the side panels and see if there are any unused connectors or accessories and check those with a meter, when selecting in reverse you would get voltage on it.
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