We received an official reply from Lexus to our query about the case of Cabir worm possibly infecting cars. The reply based Lexus' immunity to Cabir on two points (quote):

1) Navigation systems in Lexus and Toyota vehicles do utilize an embedded operating system (OS) and some degree of random access memory (RAM) that is used to store several types of information such as recent destinations, names and attributes of saved destinations, and a telephone directory among other items. The operating system itself is proprietary, however, not Symbian as these reports have alleged.

2) Although the Bluetooth interface does support the Object Push Protocol for transferring the phone book from a Bluetooth cell phone to the navigation system, this is an operator controlled event and the data cannot be exported (or transmitted) from the navigation unit.

When contemplating this answer with our mobile researchers one issue came up:
Since the Bluetooth interface supports Object Push Protocol it could be possible that when Cabir is looking for a target it might try to send itself to the Lexus navigation system (which as said before is immune to the worm) and this could cause an error message on the system, but not more than that. Before we can afford a Lexus to try this on we'll just have to speculate however.