So a little context before I ask my question.
I'm in the market for a compact subwoofer since I drive a 2003 ford f150 lariat and I was going to put it under my rear seats and I've already got an idea as to how connect it to the battery and all that. However there is a small problem, I don't know where a good ground connection would be. My question is this, is there anywhere under the rear seats that would be a good ground for a subwoofer or are the brackets that connect the seats to the floor a good ground?

 

Ground is ground is ground.
Use a wire brush/flap wheel to remove paint on the frame/body of where the ground ring terminal will go, install the bolt with the correct sized ring terminal on it.
The frame is used as a ground for most everything else already, as long as you remove the paint so it gets the best contact patch possible you are good to go.