Automotive environments can be very. . . fascinating, regarding noise. Most auto manufacturers install filter capacitors in their wiring harnesses, typically near the ignition system components (this is NOT the same as a 'condenser' as found on an old point-contact system) to reduce EMI. Most I've seen are rated between 0.1 and 1 µF.
A few common things I've found which introduce noise to the power bus are bad spark plug wires, a failing alternator (usually it's the rectifier at fault), and poor grounds (corrosion at connections to chassis ground, poor connections between engine and chassis, or even the shielding wrapped around harnesses with digital signals).
IFF those aren't the source of the interference, I think your plan to build a filtered supply into the car is a fine idea. You might want to include protection against transients.