Seat bolts which may or may not be a good ground for our high power audio goodies  

Mercedes seat bolt, this is an early 90s Mercedes and as you can see the bolt is very very corroded, I would not wanna rely on this for a good ground. It looks like it has been underwater with barnacles growing on it.
This is a Honda Accord seat bolt, it is anodized and may or may not provide a good ground, being that the carpet is between the bolt and floor, and being that the ground wire touching the metal floor is usually the real connection with the bolt just applying pressure, this could prove to be inferior.

This is a seat bolt from a suburban, it is anodized, or parkerized with a finish like sheetrock screws have. You don't use sheetrock screws for grounds do ya Gordo?  Plus, again in this case the bolt is on top of metal which sits on carpet, I would much rather find the bare metal floor and polish the paint off of it and make my own ground (in fact in this case I did, this is my suburban)
Here is where the seat bolt goes on the bottom of the truck, not the frame like you said Gordo, but into a capture nut. In this case the capture nut is spot welded to the pan, which could be a good ground IF the carpet and stuff on the top side weren't in the way...  maybe???
This is the suburban frame and the rubber bushings that isolate the pan from the frame. I think the frame is an adequate ground, but I took these pics just to show ya Gordo how far from the frame most seat bolts actually are...  ;-)