Antenna issues

Thank you all very much for the advice.
I don't like doing things just because "that's the way it's always been done", that's the reason I asked why the mirror is the best mount. I understand the mirrors are best for twin antennas, but a single should be more centered maybe?
So, I'm going to experiment with the mount where it is first. I'm going to move it to the center, away from the stack, and try grounding it with a heavy wire to the frame. If that doesn't help, I'll try the Firestick2 antenna, to see if that used antenna is the problem. If none of that helps, I'll move the antenna to the bird perch, and you guys will be proven correct.
I did look over the door, and I think I can run the coax down through where JDchat said.

 

OK, so, I experimented a bit. Pics below.
I installed a ground wire from the mounting bar to frame, no change.
I don't know what made me do it, but, I had the antenna tilted forward over the roof, mostly to lower the tip to keep it from hitting overhead obstacles, and thinking it might help the ground plane. For whatever reason I decided to tilt it to the rear. Suddenly my swr dropped below 2. I played with the angle a bit, and adjusted the whip. Best I could get was 1.7, on 1 and 40. good enough for now. Radio check on 19 got a "sounds good", but I'm parked a half mile from the truckstop. Distance will be my next concern.

 

Ok, thanks. I guess I'll get one of those Firestick2's.
I was going to move the antenna over to the oem mirror mount, but you have to disassemble the mirror to get to the stud. The broken off antenna part itself won't unscrew without holding the stud, which is inside the mirror cover.

 

Radio is an "out of the box" Uniden PC78LTX

 

This is the video on replacing the oem Volvo coax in the mirror I referred to earlier. You'll see what a PITA it really is!

 

No. In the vid it shows how the wiring is secured with clips inside the mirror, as well as under the body panel. Taking off the mirror cover is actually the easier part of the job. Getting to where the coax goes through the mounting location into the cab is the hard part.