OBD port power source
99% chance it's straight from the battery via a fuse, regardless of make or model.
80 V is the most common peak load dump voltage I have run across in the auto industry.

EDIT: 99 % of all aftermarket electronics, your average USB charger, car stereo and what have you, will simply blow up in case you loose the battery and the alternator runs of wildly. This does however happen so very rarely that few take notice. If you have low enough current consumption and you can afford to have a resistor in series, any zener diode will work wounders. If you want to meet any automotive specification, there are special clamp diodes/TVS which can withstand kW for the ms of duration at 80 V in the standard. It have very litte to do with real cars though.

EDIT2: Came to think of something. Mechanially, that 12 V may be drawn from or via the car MCU, but electrically there is nothing there to regulate the voltage, like a DC/DC converter keeping the OBDII-port at 12 V even in a 80 V load dump condition, and tha's what I assumed OP asked about.