Battery Ground Connection:
The reference 'ground' for the battery and charging system, as well as all accessories, is the chassis (body) of the vehicle. The chassis ground is the return path for all of the current supplied by the battery and/or alternator. The accessories (including the audio system) may draw copious amounts of current. Some amplifiers will draw well over 100 amps of current at full power.
There are 2 connections which must be intact or upgraded when adding any components that draw significant amounts of current. The first is the short ground wire which connects the negative battery terminal to the vehicle's chassis. The other connection is the braided ground strap that connects the engine block to the chassis.
Problems that you'll avoid:
All drivetrain components are mounted to the vehicle on rubber mounts. Since the alternator is mounted to the engine, virtually all charging current flows through the braided ground strap. If the ground strap or battery grounding wire are missing, the current will try to flow through any conductor available. This may be the throttle or shift linkage/cable. If the current flow is high enough, a significant amount of heat may be produced. It may even produce enough heat to melt the plastic liner inside the cables, causing them to seize.
Vehicle Floor Pan:
The vehicle floor pan is, on many vehicles, a single piece of stamped sheet metal that runs from the trunk of the vehicle to the firewall. It is the best return path of the vehicle's chassis. When you ground any electrical accessories (including your amplifiers), ground them to the floor pan. Do not ground anything to braces or the wheel wells. These pieces are generally connected to the floor pan with a few spot welds. If you ground to any spot welded sheet metal, you will have more resistance in the return path. The floor pan will have less resistance. Any extra resistance will lower the voltage at the amplifier and your amplifier will produce less power.
The seat belt bolts are rarely a good place to ground your equipment. Sometimes there will be a thick tar like substance (used for waterproofing) that will prevent you from getting a proper ground. Even if there is no sealant, the connection may be less than perfect because the hardened steel seat belt bolts (relatively high resistance) go into a hardened steel nut that's pressed into a piece of stamped steel that's spot welded to the bottom of the vehicle (not exactly a great electrical connection).