As my brother suggested, I have painted the inside of the body in satin black (Humbol enamel 85). I couldn't get to all places on the inside body, but all places I could access have been painted black. The only place I didn't paint is the inside of the cabin. That is left unpainted so far. The paint on the inside of the body made me want to paint the axles as well. I think they would look better if these were also painted. I might do that at some point.
I learned that the CC-hand mirrors aren't very durable. I have broken both without much effort. I'll fix them with some brass rod. That should make them last. Then I have finally put Velcro on the body mounts, so now I can fix the body to the chassis. I needed the inside body paint, before the Velcro could go on.
Then I have received a light and sound module. So this will probably be one of the next things I will look into.
Here are pictures of the inside of the body and the broken mirror: