What is this two-lead component across an AC input?
On the input to the PCB from the alternator, this large Diode-case-like component appears to be a bi-directional (due to AC, and before the bridge rectification) Transient Voltage Suppressor (TVS) diode. 

These act as surge voltage absorbing components, which dissipate any voltage above the clamped voltage (as heat)  by redirecting the current to the return path. 

These are very important for automotive style power supplies like alternators, because the voltage output of these can have seriously high spikes. A car may output a "steady" 14-16V while the engine is running above idle, with spikes in the 40-60V range occurring frequently. 

Without these voltage clamps, the diode bridge (probably rated to 40V) would probably fail, any further capacitors or CMOS devices would instantly fail from destructive voltage breakdown. 

Inside the package it looks like this (ignore the default device names shown..):







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