An interesting vehicle.
Quite by accident, I found this photo today which shows in the rear files several vehicles that match many of the ID features of your puzzle ambulance. The design of the cooling louvers in the side of the motor hood, the style of the wheels, the windscreen wipers etc. etc.
The photo (from ebay), was supposedly taken in 1939 and as many of the vehicles are early war German, I assume after the battle of Poland was over. I did a pretty good search of German/Austrian vehicles of the era as they were used in the invasion of Poland and came up with nothing. I also did a search of my Polish files and came up with nothing.
Still, I now think it may be some sort of Polish marque since there are so many of them and they look to be in very good condition.
Am I closer to the truth??
Bill