12PCS Automotive Ground Straps, 10/15/20 Inch Tinned Copper Braided Grounding Straps with Ring Terminals, Automotive Ground Cables for Car, Truck, RV, Boat, Motorcycle

Anodization / paint / powder coat is an electrical insulator. Grind it off the mounting plate where the ground side of the antenna mount and around all bolt holes, and then apply no-ox-id on and around the exposed metal and bolt threads. It would also help you to run a grounding strap, as they are far better at grounding an antenna than simple wire. I would also use different coax, rg-316 works quite good on vehicles with the short runs and is very easy to fish theough small holes. Also make sure you've got the mounting stud on correctly.