It turns out that a simple
Bluetooth hack can unlock a Tesla or break into all kinds of devices. It's a variation on the man in the middle attack known as the relay attack, and takes advantage of assumptions built into the Bluetooth Low Energy standard.
Obviously this has all kinds of nefarious uses -- but even as I'm reading it, I'm wondering whether my protagonists in a "light has gone out in the city on the hill" alternate America could use it to fight the tyrants as well.