Best way to wire a capacitor for car amplifier noise suppression

A cap won't fix ground loop hum or alternator whine the way people sometimes think it will - those need proper grounding and shielding instead. What it *does* help with is voltage sag when your amp suddenly draws current, which keeps your headunit and other electronics from dimming or cutting out.

When I wired up a decent amp in an older car, I learned that placement matters way more than most guides suggest. The cap needs to live as close to the amp's power input as physically possible, not just near the battery. I ran my positive lead from the battery to a distribution block mounted under the rear seat (where the amp was), then connected the cap right at that block with maybe two feet of cable - way shorter than what the battery-to-cap route would've been. Ground it to the chassis through a bolt with a star washer, cleaned-off metal, no paint. That short, low-resistance path is what actually lets the cap do its job during transient peaks.

The wire gauge matters too. Most people grab something random, but if you're running a thick power cable to the amp (like 4 or 8 gauge), use the same gauge for the cap's positive and negative leads. Mixing thin wire with a thick main power line defeats the purpose since the cap can only react as fast as its connections allow. Quality terminals and a solid fuse holder on the positive side keep things safe, especially if something shorts inside the cap itself - unlikely but worth the five bucks of hardware.