Well ideally you want to ground to a giant piece of metal that can take a hit and is separate from the rest of the vehicle so ground feedback doesn't occur.
I just used the strut tower in the rear of the trunk. As long as my rpm's are up I don't have issues with dimming and that's while seat heaters are going and all the other electronics going too.
A lot of people just find random bolts and attach to that which is a bad idea. Sometimes there is a type of zinc alloy used for the seat bolts that will cause noise in your system. I don't know the reasoning I just know it does. Other times people grab bolts that are attached to a thin formed sheet but the bolt isn't making much contact with the metal at all. It's ok to use these locations but grab a large washer and remove some paint for better surface contact. The other issue people predominantly have is when they use two grounding locations that are touching each other. If you have noise in your system using this configuration it's the difference between two unequal grounding points interacting with each other.
A lot of times it can be trial and error. Another approach people take to avoid multiple ground points interacting is creating a single point source ground that extends back to the battery. Radio, amps, everything grounds at the battery in those setups. But upgrading your grounds in the engine bay are a very good idea at that point. I added a 8awg wire to my stock grounding at the battery. It's a little long but it's more of a "feel" accessory I guess?
