What is the modification running from the back to front and over the doors on this car?  Some sort of alternative fuel setup?  The picture was taken about 1943.

I do believe it's burning Car, don't know how it worked though LOL.

I think it's a set-up for a wood gas generator instead of normal fuel, see the sacks of firewood on the roof rack.

Read an interesting online article "Cars that Run on Trees".  Said that in 1944, at least fifty Tiger tanks were retrofitted with wood-gas units. I guess the use of wood for fuel was more wide spread that one might think!

The German term is "Holzvergaser". I have read somewhere that 3 kg of beechwood were needed to replace 1 liter of gasoline.

Couple of wartime Volkswagen examples. Sorry for the poor photo. Taken from 'The Beetle', Keith Seume, author.

It’s been many years since I read it, but wood burning cars are mentioned in the book Is Paris Burning? I thought it was an interesting concept. 

It already was a somewhat normal modification before the war, so not a wartime invention