Re: "A direct causal link is difficult..."
"then the increase in thefts would be disproportionately focused on certain makes and models of vehicles,"
Go look. It is.
In particular, high end vehicles loaded with electronic control and access systems are being increasingly targetted. Up until recently thieves were conducting targetted burglaries (and in a few cases, armed home invasions) to get the ignition keys but such vehicles are increasingly being stolen without the keys being touched.