Antenna issues

Setup:
07 Kenworth W900
Cobra 29LX
Dual 4ft Firestiks on mirror mounts

I need some advice. I had a radio burn out on me. I installed one that I had peaked and tuned earlier. It was showing an antenna warning when transmitting. I disconnected both antennas and ran a single cable (18') to the left one, got the same warning. Ran it to the right one and no problems. I figured the left one must be bad. I've been wanting to go to a single antenna anyway, so I ran the new cable permanently to the left side and switched the antennas. Left the one on the right side just for looks. Tested it again and same problem. Checked the swrs just with the built in one and it was over 3. Frustrated, I replaced the antenna and mount with a Francis Hot Rod. Now I have new all the way; radio, cable, mount, and antenna. Same result. Thinking ground, I ran a short temporary wire from a bolt on the cab to the antenna mount. No result. Now I've run a temporary new cable back to the right side old antenna. It works just fine. Swrs are where they belong and no warnings. Does this sound like some sort of ground problem? Do I need to take my mirror off and do something to get it back?
Let me know what you think. Thanks.