Cascadia CB antenna mirror mount grounding

I've got my 5ft Firestick2 mounted using one of those U shaped brackets. My SWR reading is showing just barely above 2.

I noticed that it goes down to below 0.5 when I have the door propped open.

I thought maybe since my cable is running through the door gap, the cable is getting pinched somehow, but it looks fine to me. Then I thought maybe it's just changing the position of the antenna as I'm opening the door and it's reflecting less off the cab, but the SWR doesn't go up and down gradually, it actually drops right when that catch mechanism locks in, the one that keeps the door from closing on it's own.

This leads me to believe that maybe the door doesn't have a very good ground connection to the rest of the truck, and this catch mechanism is improving the ground from the door to the body of the truck.

Does anybody have any thoughts or suggestions to keep my SWR lower with the door shut?

 

Can you elaborate? What does one of these straps look like, and where can I get one?
Attached 2 photos showing SWR with door closed, and door open. Quite a big difference.