Don't do it yet. you have your antenna mount incorrectly assembled.
There should be an insulator that keeps the antenna separated from the mount itself. The mounting bracket DOES need to be grounded, but the antenna needs to be isolated from ground. The antenna works as the positive part of the system, and the vehicle works as the negative part of the antenna system. The radio put out both a positive and negative PHAZE of the signal, and the center lead of the coaxial cable is the positive, and the shield of the coax is the negative/ground.
So, you should have the antenna isolated from the mount with a plastic dielectric washer/spacer piece, and the center lead of the coax should connect to the antenna, while the shield part of the coax connects to the mounting bracket, which should also be connected to the vehicle frame via a ground strap, or directly mounted.