Mercedes Unimog S404 Ambulance ICM 1:35

Let's start with step No. 4: part D45 is not symmetrical. Check the orientation of the red marked parts. 

As always alignment of the frame is crucial for the fit of everything else. I suggest not to glue part D58 but only to put it in place. Perfect orientation can then be achieved when mounting the gear box.  

I recommend not to glue engine and gearbox together at this stage. The attachment faces between the two allow quite some range which might cause you problems when installing this complex to the frame. Installed separately later you can wiggle the two parts nicely together. Due to a misalignment in this step I had to separate engine and gear box again. 

The lower parts of the tanks have to be mounted on the supports in a single orientation. Double check their position on the supports and only then glue the top part onto it.
I hat to remove the intake again and transfer it to the other side - leaving a hole at the back of the tank - because I malrotated the top part by 180°. 

I suggest to align parts B18, B29 and B1 in one go. I ended up with a slight warp which made mounting of the dashboard difficult
Check part B25. It has a little knob at the lower edge. I mistakenly removed it, thinking it was a sprue gate. It is not. This led to the next mistake: mounting the front grill upside down (yes, it fits this way too). Correcting this was possible but required sharp tools, brute force, foul language, white putty and careful sanding. 

Parts C1-5, C1-8 and C1-20 constitute this box. When mounting the round tube gaps remain.  

This looks ridiculous, so I closed it with a sheet of styrene. This is wrong. The gaps are to be seen at the real thing too. So, no need for extra work