>>layer8+(OP)
Completely untrue. If some clever engineer or consortium of engineers designed a 3D-printable car for 3D printing-and-manufacturing companies to make then it surely would exist. If you buy one from a Ford dealership you'd be getting the Ford-branded version which may have their own tweaks to the design.
It makes perfect sense to me that the big carmakers could get together some day and develop a handful of car platforms that all their cars will be built upon. That way they can buy the parts from any number of manufacturers (on-demand!) and save themselves a ton of money.
They kind of already do that, actually =)