Also, it's recommended that you use the same size wire for your power as your ground. You can't expect good electrical flow if the ground wire is smaller than the power.
Think of it as plumbing, starting with a 1/2" pipe and reducing down to a 1/4" and then back to a 1/2", the 1/4" is going to restrict the flow of the water.
I would create my own grounding point, I pulled the carpet back away from the trunk floor, behind the back seat, sanded to bare metal, drilled a pilot and screwed my own screw, just make sure not to drill through any wiring or lines on the underneath side of the car.
Remember, keep your ground wire as short as you possibly can, figure out where you're mounting your amps and find a point close to the amps. I try to keep my ground wires under 2 foot long. Shorter wire equals less resistance, better electrical flow.
