It’s not as simple as that. They will probably integrated the system into the ignition. Disable the black box and the vehicle won’t start.
Nothing that allows physical access is unhackable. Witness DVD encryption.
I'm not a "car guy" and I've never been interested in tinkering under the hood, but I would get under there to replace these boxes with dummy boxes that feed "OK" signals to the rest of the car computer network.
Jailbreak and re-flash your car’s firmware.