I'm guessing this is some strange WW2 alternative gas storage balloon

What can be seen on the roof is the fuel tank of the vehicle - a balloon filled with uncompressed gas.
The vehicles consumed 'town gas' or 'street gas', a by-product of the process of turning coal into cokes (which are used to make iron).

Wood gas cars are a not-so-elegant but surprisingly efficient and ecological alternative to their petrol (gasoline) cousins, whilst their range is comparable to that of electric cars.

The fuel used for gas bag vehicles during the World Wars was generally not compressed and had a much lower energy density than LPG or CNG. To replace one litre of gasoline, two to three cubic metres of gas was needed.

The gas storage bags were made of silk or other fabrics, soaked in rubber (Zodiac was one of the manufacturers). These bags were (and are) much cheaper and easier to build than metal tanks. They could also be repaired in a similar way to bicycle tyres.
http://www.lowtechmagazine.com/2011/11/gas-bag-vehicles.html

Wood gasification is a process whereby organic material is converted into a combustible gas under the influence of heat - the process reaches a temperature of 1,400 °C (2,550 °F). The first use of wood gasification dates back to 1870s, when it was used as a forerunner of natural gas for street lighting and cooking.

At the end of the 1930s, about 9,000 wood gas vehicles were in use, almost exclusively in Europe. More than one million producer gas vehicles were used during World War Two.

During the Second World War, coal and peat were also used, but wood was the main fuel.

The use of wood gas limits the output of the combustion engine, which means that the speed and acceleration of the converted car are cut. Wood gas consists roughly of 50 percent nitrogen, 20 percent carbon monoxide, 18 percent hydrogen, 8 percent carbon dioxide and 4 percent methane.

Nitrogen does not contribute to the combustion, while coal monoxide is a slow burning gas. Because of this high nitrogen content, the engine receives less fuel, which leads to a 35 to 50 percent lower output. Because the gas burns slowly, a high number of revolutions is not possible.

The woodmobile caused severe deforestation in France during the Second World War

http://www.lowtechmagazine.com/2010/01/wood-gas-cars.html

http://anyskin.tumblr.com/post/88969077682/historicaltimes-gas-driven-taxis-on-the-streets#notes