240, You have some good points and present them well instead of just criticisms..........I like your reasoning.

Engine performance is more important to me than audio quality, so I run
actual wires, and clean and grease the places they are bolted to, to insure good contacts. I will soon be running ground wires to the header bolts (next to the spark plugs) to insure the cleanest spark possible
across the spark plug electrodes.....and I run real wire for spark plug wire instead of other stuff. I have the battery close to the engine, over the front wheels for front end grip....when I drove rear drive cars, I put the battery in the trunk......

and I understand what you are talking about....there is induction current
around any wire, and it will affect audio.....using the chassis as ground doesn't require a shielded wire. a little like a radio antenna uses a shielded antenna wire to keep induction noise from being picked up on the radio.

2 reasons that MFG's would use no wire, when they can use the chassis to conduct current......
1. If the quality of the sound system is good, people's perception of the quality of the car is good. Other "quality perception" examples would be the use of rubber nards in critical chassis spots to dampen out vibration harmonics.

2. Economics.....When millions of cars are produced, any cost that can be saved per car is equal to millions in profit, and wire costs money ......other examples are where they use plastic snap-in screws to hold something instead of metal screws.-Bob