I have found some Polish data on this subject showing following figures (I expect they present only civilian vehicles) in following years:

1928 - 22000
1929 - 31000
1936 - 23000
1937 - 24000
1938 - 25000
In 1938 there was also 4.000 trucks (included in the above figures) and additionally 18.000 motorcycles.

I have two questions:

a) are those data correct?

b) if they are correct: could you please explain what was the reason of a drop between 1929 and 1936 (could somebody share with exact data for the particular years between 1929 and 1936) ? Is it just Great Depression? Romania did moderately well in this period (at least what available data on the industrial output shows - much better than Poland in this period for example and only 1932 the industrial output went before level from 1928) so I am a bit surprised. From the data I have (based on Polish statistical books from pre-war period) it is only Poland and Romania who suffered such drop in number of the vehicles in listed countries.

PS I have posted similar thread on Axis but it seems that this forum is much appropriate place.