The radio works as expected with the magnetic base antenna. However there are no instructions for the mounting of the phantom antenna.
The 3/8 stud is too long for just a sheet metal roof. Start with a 3/8” hole. Remove the paint around the hole on the underneath side of the mount. You will need to add a stainless steel washer and/or a star washer under the sheet metal. At least the star washer to ensure a good ground. Discard the nylon stepped washer (used for brackets with 5/8 hole). From the topside, screw the base onto the stud. The antenna should now screw on and seal off against the sheet metal roof.