That's a Federalism/full-faith-and-credit argument, which is fine. One distinction I draw is that car ownership is a benign and legally-recognized activity in all 50 states, whereas slavery was illegal in a state a slave would have fled to. (However, my recollection is that Dred Scott tried to deal with that by inhibiting the ability of a free state to grant any status to a slave that would affect the rights of the owner to his property.) The fact remains, southern leaders were perfectly happy to avail themselves of federal power when it suited them - so arguments about the poor-downtrodden-South-just-wanting to be-left-alone are a little specious.

I can understand why southern leaders pushed for a federal mandate to help retrieve runaway slaves. Slave-catchers got less and less cooperation over the years from non-slave state citizens who increasingly found it morally repugnant to assist agents of slavery (though those citizens may not have wanted the ex-slave to move in next door). If a stolen car shows up on my driveway, I can be reasonably certain it won't be flogged or crippled when it is returned to its owner.