A car is a hellhole for electronics to live in. I read an interesting Intel article some time ago, and the WORST thing that can happen on the +12 line is a "alternator dump", where all of a sudden load get shed quickly, like, from turning your headlights off or the A/C relay kicking on/off (high-current deltas). This puts a spike on the +12 line. Even the battery (acting like a giant capacitor) can't handle it entirely because it can't handle really short pulses (this is one of the reasons to NEVER run your car with no battery or a dead battery).

I THINK (don't know for sure) that the OVP is like a "crowbar" circuit on a switching power supply - that is, when voltage goes over, say, +16 volts, a big SCR (a cheap, high current semiconductor) latches in and presents a dead short - which will then deliberately blow the fuse (and maybe the semiconductor, which is designed to fail shorted). Depending on how the circuit is made, the circuit might NOT latch so the circuit can survive short overvoltages.

This "cheap" (in the relative term) part sacrifices itself (or its fuse) for the sake of other stuff.

Of all the magic MB modules, this one is the most promising to reverse-engineer!