Just capacitor could be not enough. Look there how old transistor ripple filter is done :
**broken link removed**
(r1 r2 c1 Q1)
Never worked with car supply bus , but suspect that spectrum of ripple and spike is too broad so C1 must be shunted with some capacitor enough to damp high frequency parts of noise spectrum.
For 100 protection put tvs to Q1 emitter to ground with voltage more than at filter output (defined by R1 and R2 ratio) and low ohmic resistor between this scheme and filter input - to prevent over current through Q1 .
I dont take into account current your device will sink.
Yet you can use coil of course instead of electronic inductor above.