A "noise filter" is just a big capacitor.

A cap basically is a filter that a) blocks DC (like from your car's battery) from passing through, b) allows AC (like the electrical noise generated by your alternator) to pass through, and c) will store an amount of DC charge (this is why you don't poke around inside the back of TV sets or tube amplifiers unless you know what you're doing!).

The negative (-) side of the cap connects to ground.

Wire the + terminal of the cap to the output of the alternator, or the BATT terminal of your ignition switch. I don't recommend putting it right on the input of something like the tach or ignition system, because it stores a charge, which can take a couple seconds to bleed down.

I like to see filters "upstream" or "before" the ignition switch, so that if you switch the ignition off, that pesky charge doesn't keep the igntion hot for that extra second or two. (Of course, there may be enough load so that it drains the cap in a fraction of a second, depending on your car's wiring... but I wouldn't want to promise you wouldn't have that momentary 'run on'.)

This way, everything downstream of that filter cap has (most of) the noise bled off the 12V power coming to it.

Crane sells a 16V 38000 uF cap; available online for $35. Mouser sells that same value cap for $19.45 here.

I'd like something with 20+ volts worth of insulation myself; 16V is cutting it kinda' close when a strong alternator output is crowding 15V. I'd expect any cap 22000 uF or more to be sufficient; more doesn't hurt, but costs another buck or two. Here's another.

Hope this helps. If not, PM or email; I'll try to help y'all out.

-Bill