Is the sleeve supposed to connect to GND? I don't see that connection anywhere. If it's missing, I can see how that theoretically could blow up a BS170.
Are you sure the ring, when shorted to the sleeve, will have a current limit that is below what the BS170 can handle? What do the specifications of the Sony radio say about this?
Keep in mind the automotive environment is fairly hostile for electronics and you generally need to think about protection of components in various ways. Your circuit doesn't seem to have any kind of protection anywhere.