After buying some different window vipers I finally found out which would work: RC4WD Mojave (Tree Z-B0025). Unfortunately only one of the vipers in the tree can be used, so two kits are needed. It is not a perfect fit right away.
The blades have to be shortened and I did some twisting of the viper arm. To do the twisting, I had to heat the plastic in the places I wanted to make a twist. Careful with the heat as it can easily be too much. To capture the looks of the Danish army UNIMOG I had to paint the viper arms. Here are some pictures:

Now I am still looking for a way to make door handles and a schnorkel