The filter capacitor cleans up the ripple to give you a flat DC supply. There is an equation for it (ripple and load based), but since the supply is DC, not AC the ripple is minimal. A 780 amp hour battery is an amazing cap, so a 10uf input cap for an auto system is fine.
As Alain alluded though, higher frequency stuff sits on the DC that is VERY bad. Those intermittent wipers keep firing a DC relay and kicking pulses of CEMF on the line. so, add a .001 and a .1 cap in parallel with the 10. Also, I'd put a .1 ohm resister in series with the regulator and hang a 25v zener on the regulator input. The output needs a 1uf to prevent oscillation FB. A typical 3 terminal regulator can handle up to 35vdc, though only for a short time without heat sinks.