I'm not an audio guy, for some reason the strut bolt doesn't sit well with me as a good ground. My thought on that is.. struts aren't 100% weatherproof, the bolts corrode, the area inside the car is known to rust and that's not a good long term ground.

I went to one of the main achors for the back seat. It's interior, clean and always dry there. Plus is was super close to my amp.