There was a year over year drop. I use Ward's:
http://wardsauto.com/keydata/Ward's has May auto sales down 3.6% YoY for May. Light trucks are down 4.2% YoY.
At the beginning of the year, light vehicle sales were really upholding the US economy, and now that seems to have reversed like everything else.
Some of this is from short inventory on vehicles affected by the Japanese quake/tsunami disaster, but if you download the spread sheet at the link, and if you look at the sales by company, you'll see that even US domestic sales fell. Therefore I would say light vehicle sales are structurally down.
This does mean that the May retail sales report will be bad; auto sales are a large component. When inventory is reported, Ward's will update the above link and we can tell more about future production plans.