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An aftermarket head unit connects to the CAN bus? The aftermarket head unit I installed certainly doesn't. Are you sure what you are saying, which is true for OEM units, applies to aftermarket ones?

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Can't vouch for all car architectures, but in most cases the head unit is QM and safety domains are usually segmented from each other. So even if the head unit talks CAN (it needs to get car data somehow) it will only communicate with the rest of the car through a gateway that will not allow it to take any dangerous actions.

Mine does. Its dashboard shows fuel level and a bunch of other things and I can bring up a speedometer/rpm app.

I believe the connection exists because the steering wheel buttons/iDrive talk to the original head unit over CAN.

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