Updated: Mar 30, 2021
So in 15 years experience I love and hate to admit that I have probably seen it all when it comes to grounding an amplifier. Both good, and very very bad. LOL So here are the basic points to why we need to ground an amp: Your amp takes power from the battery and then the power loops back to the battery through the ground path. So with that being said, your ground is every bit as important as your power wire. I see a lot of mistakes made, but the most common and overlooked mistake is when someone runs a large gauge wire to the amplifier, 4g for example, but then leaves the factory ground wire (8-6g usually) on the battery. The thing to realize is that the factory wiring and factory ground was only really designed to power the factory installed accessories. So that is why they use a small ground wire on the battery. Totally OK from the factory, but not good enough for your 500watt amp. The #FAMway to do the install would be to add an additional battery ground the same size as your power supply wire. This way you can complete the loop without bottlenecks. This is the first step of the big 3/4 upgrade. Upgrade your ground wire from the battery to the frame/chassis of vehicle. I Also like to run an additional ground to the engine as well. Speaking of bottlenecks, that brings me to the subject of making your grounds. Think about the gauge of wire and the current running through it. 4g for example, but if its being bottlenecked through a loose or a poor amplifier ground then you will get a voltage drop which is bad mmmmmk. A lot of times people will mistakenly ground to the seat belt bolt because it is structural and strong. Sure it may work, but that does not always mean that it would be the best electrical ground. My guess is that grounding next to the seat bolt and not on the seat bolt itself would be better considering the glue that is often used in the threads of a seat belt bolt is not a good conductor. You want to ground to a good electrical spot so any glue or paint between the electrical conductors would actually work against you. Now you will see in the pic below that you can still have a poor ground even if you don't go through the seat bolt and do a poor job at removing the paint. The self tappers used here are small diameter and time after time I have seen them loosen up in a few months and that can, and has, damaged many amplifiers. Usually, at that shop, after they swapped the amps a few times the manager would ask me to get involved and double check everything. Sure enough, almost everytime, it was the result of a loose ground.
So for my grounds I use a very nice zinc grounding lug. Instead of what I see lots of guys do with cheapo self tappers and lots of them, predrill the 1/4" hole and then my zinc ground lug cuts its own threads. Much larger and stronger threads that can actually provide some clamping force on your ground and not strip out easily from being overtightened. Notice how the paint is scraped away to the bare metal and the ring terminal is bolted through to the great metal ground. This may not be my prettiest work, but it is electrically solid. Since this (old) picture was taken I have upgraded my tools and techniques to use a better wire brush for cleaning up my grounds. Please check out one of my new favorite tools in the link below: https://www.franksaudiomotive.com/product-page/centering-wire-brush https://www.franksaudiomotive.com/product-page/zinc-thread-cutting-ground-bolt https://www.franksaudiomotive.com/product-page/copper-ring-terminal-4-gauge-5-16-inch
Above you can see a vehicle I worked on that had a very poorly connected ground where the snake bite looking holes are. I predrilled a new hole with a 1/4" drill bit, used my centering wire brush to clean the paint off to bare metal, and then bolted the copper lug in with the zinc grounding bolt. Afterwards I put some silicone and tape over the old holes to seal that back up. Below you will see another example of another ground I did with this kit using a customer supplied ground wire and ring terminal. Same basic process for an excellent ground.
If you suspect that you might have a bad or a weak ground, there are tests you can run to diagnose and determine if that is the case. The test is called a voltage drop test and is very easy to do, and very helpful to measure ground potentials. You can use this technique shown in the video for a starter circuit, to diagnose your amplifier or rear battery install. The ideas taught here can easily be used on your audio gear to determine if your ground is good enough. No worries if it tests bad, thats what learning is about, now you can buy the right parts I have linked to in this blog, change your ground, and test again. https://www.youtube.com/watch?v=qPpHRuddhh4