I've been having some background noise with my X710BT and I'm wondering if it has to do with how I grounded wires. There are ground cables on each harness, i.e. the harness that integrates into the car and the harness that plugs into the Avic. I grounded these cables to the steel chasis of the car. Should I have grounded them onto each other, like literally tie them together and wrap in electrical tape?

yea the ground wire in the factory harness, is the wire comming from a body ground for the radio, it should be connected to the ground wire of the radio, however as long as the radio it grounded to the body of the car, then you should be all good unless the spot you used is not a good ground(has a high resistance to ground)