Skip, even the diode won't protect you. In fact, with our automotive parts we design many of them with undervoltage lock-out (and other protection schemes) to prevent users from damaging parts doing exactly what you are doing.
My advise is
never power up an automotive system under 12V. You can create walking wounded devices as well.
On my current project, I'd say 30% of the design is the protection scheme (undervolt, short circuit, etc). You're welcome
