The metal frame of the car is the other half of the circuit. Problem is two fold. Steel (iron) is a rotten electrical conductor (or of heat for that matter). On the Corvette which has a plastic body that doesn't conduct electricity at all, you have to use a separate copper ground wire for every circuit. This doubles the number of wires in the wiring harness. I learned automotive wiring and bought most of my wiring tools trying to rewire a 1959 Corvette roadster that I bought as a basket case (failed restoration) back in 1965.
With your steel body that was spot welded together you have to deal with corrosion. oversprayed paint and just poor connections that have vibrated loose. To simplify things I recommend running a separate ground wire from the battery negative terminal into the car's cabin and screw it into the steering collum support brace. Then attach all of your grounding wires to that one wire for a better ground.
Since most engines have been removed from the car at least once often the grounding straps for the car body are missing because they are too much trouble to reatach to the engine at the bell housing. There were two flat copper braided ribbons that grounded the car's interior, and the car's frame to the engine. The engine was grounded to the battery originally by way of the negative battery cable that could now be trying to ground everything through a shiny fresh coat of insulating plastic paint.
This is also a real problem for the starter motor as well, as it has to ground to the block to work, but is frequently insulated by Chevy orange paint on the block due to a fresh rebuild. Chevrolet never assmbled and painted a motor the way restorers do it. the paint was slathered on from the top; painting everything attached to it. They also missed painting many places of the block entirly because they painted the engine with the ehaust manifolds bolted in place.
You need to measure the actual resistance of ground circuits to verify that they will work (not reduce the voltage to zero by having more resistance that he car voltage can over come). This is why the inside joke is that eltrical engineering is as easy as PIE; which is Ohm's law, used to calculate if your voltage can over come resistance or if you wiring can handle a load. A continuity checker might verify a circuit is complete, but it doesn't indicate the actual quality of the circuit.
If you are thinking of a heavy duty amplified sound system then you are probably already aware that the factory wiring isn't up to the task. (I once saw a Volkswagen type one, aka "the beettle",filled with speakers and with it's trunk stuffed with a 24,000 Watt peak to peak amplifier in a sound store back in the sixties. That rated Wattage would have require more power than the engine could produce to run if the Wattage claim where true). Same applies for an electric fuel pump or an electric fan. Modern power consuming devices require a modern one wire alternator to feed them. This is why an A/C equpped car had the higher rated alternator and bigger battery (heavy duty electrical option) from the factory, to keep up with the blower motor running all the time.