During an air filter change as part of regular maintenance I stumbled upon this:

Thinnest ground wire ever.

No problem, I had some extra 4-gauge power cable from my amplifier install, so I bought some ring terminals and made this:

New ground cable up top, OEM wire on the bottom. Big difference!

…and installed it. Took all but 5 minutes after the soldering iron was hot. Now the engine block has a much better ground:

Proper ground cable.