But in the case of cars, we know they don't sneak off to do the nasty after being put in the garage at night, so we know they don't genetically inherit design features from previous generations of automobiles. If they did, and we also saw no observable evidence of the intervention of automotive designers implementing design changes, it would be reasonable to conclude that the similarities between the cars indicated a common ancestor automobile.
Its not just the functional parts (only about 3%) that are shared. All of the genetic garbage and broken stuff is as well.
How do you explain that? A designer who would (a.) Make all of these mistakes in the first place (95% of the time) AND (b.) Put them in organisms in such a pattern as to explicitly spell out common descent is either stupid, malicous, deceitful or all three.