E.g. Boston had 1,575 reports of auto theft in 2012, compared with 28,000 in 1975; Massachusetts had 242 murders in 1975, and 121 in 2012. (a 56% drop in homicide rate, as population went up 14%)
Are there any aspects of the crime that make it less appealing? Electronic counter measures too good? Price of replacement parts no longer carry a premium? Too easy to get caught?
I would bet that the pervasive use of electronic records has something to do with it, too. According to this 1979 report from the Nat'l Assoc. of Attorneys General, in the 70s there were a lot of paths to retitling a stolen vehicle back then, which along with the the rise of chop shops and easier export of stolen cars, supported a large stolen-car economy: https://www.ojp.gov/pdffiles1/Digitization/59904NCJRS.pdf