I'd start over with basics.
Verify continuity of the center conductor from the back of the radio to the tip if the antenna.
Then continuity and resistance od the outer shield at the back of the radio to a good ground point on the body of the vehicle.
Verify there is no continuity between the center conductor to the outer shield at the radio. Do all without disassembling the antenna just remove the cable from the back of the radio.
Resistance in the ground should be under 5 ohms. The lower the better.
If you do not have these continuities where you should there is something broken, damaged or installed improperly.
You mention your antenna is on the rear tailgate? The hinges make poor ground conductors and cause many SWR issues.
"Never look down on anyone unless your helping them up"- Jesse Jackson