[ngneer]

Allow me to offer a different opinion. There is little sense in applying logical security when physical security is lacking. CANBUS should not be accessible by taking apart headlights. Communication buses must be protected from physical access, i.e., trip the alarm system or disable the car upon unauthorized access. There can be no logical security without physical security.

[0x457]

It would be very hard to make CANBUS inaccessible from headlights, since that what controls it. However, the headlight shouldn't be able to tell the rest of the system that the key is in the car.

[ngneer]

Logical compartmentalization like you suggest is a fine approach, but even better is to not allow physical access. Unless the car is in maintenance mode at the shop, the chassis should be sealed tight. Maybe the manufacturer decided to favor headlight maintainability over theft prevention, or was simply oblivious.