Hello Klaus welcome.
The chassis no. can be found on the chassis beam under the seat.
With my 4/4 it was on the beam in front of the right seat.
At the +4 behind the right seat.

Paul

1990 4/4 1.6 Ford CVH Sold
2014 +4 2.0 Ford
2005 BMW Z4 2.2 6Cyl Sold