How to stop Car Alternator audio noise / buzz?
All old car radios had filter components on the incoming 12V rail. New stuff probably has some filtering as well. Car audio retailers sell inline filter chokes to quiet noisy installs. Some cars are noisier than others. Connecting straight to the battery via a fuse is generally regarded as the way to go for lowest noise. The reasoning being that the car battery forms a natural filter.

The old car stuff had an inductor in the low milli-herny ballpark and an electrolytic cap of around 1000 microfarads. These days you could try a cheap 500 micro-henry ferrite bobbin core inductor with a 2200 microfarads electro capacitor.

The reasoning for my value changes is that the old chokes were laminated iron and now expensive and the bobbin core ferrite coil can be had at only 500 micro-henry at small size and reasonably low DCR. The cap value increase roughly makes up for the inductance decrease. This stuff works on car radios so it should work for you.