I'm installing a 600W RMS amplifier in my trunk and can't decide on a good place to ground. I have some pictures below. You will give to excuse the lack of lighting. I have limited daylight to work with this season so I lit up the area with my flashlight.
There are three holes in this piece of sheet metal located behind the side trunk liner. I could sand away the paint and put a bolt washer nut through it. The only problem is I'm not sure if the two piece of sheet metal have metal on metal contact. If they don't it isn't much of a ground.
This is also behind the side trunk liner about 6 inches from the back of the rear seats. I'm not sure how good of a ground it is and what it is connected to (unless I take off the nut and the bolt is holding in something heavy and it drops...unlikely but I'm pretty terrible with mechanics).
I'm thinking this might be the best option. I have no idea what these yellow plugs are for (maybe if someone has a clue let me know), but if you pull one out, there is a nice hole that I could find a bolt for. It looks like I have about three inches of depth before another layer of the body of the car (so I should be able to avoid rust). I would probably find a nice size bolt, a washer, and a lock washer. Sand the paint away. Put the terminal against it. Would a small spot weld to make sure it doesn't loosen (but not so much that I can't pry it off) be overkill?
There are two bolts. One is for the base of the seat (nice bare metal, but I can't imagine it being a sufficient ground). The other one I believe is for the seat belt bracket. No idea if there are consequences for loosening this bolt.
These bolts are next to the tire well. The closer bolt would be nice metal on metal if sanded. I have no idea what it goes to and if it is ok to remove temporarily. The rear bolt is so-so looking.
Let me know what you think.