To me, grounding will work for older cars, where the wire resistant have sort off gone up...ohh ya one more thing, I've actually asked a shop owner of car accessories shop and he told me that grounding cable doesn't really give any difference, at least for his perdana v6. He'd advise me not to install thing kind of fancy stuff.
For example, some old car will have like signal light problem, or brake lights problem because earth connection not good. Adding earth will solve the problem....something like that....