The truck is a second gen dodge. The antennas are dual firestiks, the trim to tune types. I have them mounted behind the cab, about 6 inches back in the bed. Before that they were on the mirrors, same problem. I moved them because I thought it was a ground plane issue but I’m assuming not since the same thing is happening with them behind the cab.
My guess is it is the t-splitter causing the problem. You really need to have a proper cophase harness.
ALSO:
If those antennas require a ground plane, neither the mirrors or the bed rail will give you a good ground. You would be far better off with a single antenna in the center of the cab roof. If you insist on mirror or bed mount, you need antennas that do not require a ground plane.
Make sure you have the correct coax - 75 ohm for duals. 50 ohm for single.
Also make sure the studs are not touching the mount - are you using the nylon washers that are supposed to be there?
If less than half of the antenna is above the roof of the cab, or if it has a coil that is below the cab roof, it will negatively affect SWR.
Check all your coax for continuity - any worn or crushed sections of coax can cause high SWR.
Keep in mind that even if you do get the SWR down, pickup trucks aren't wide enough for effective cophasing of CB antennas. Your dual setup will give you range directly to the sides of the vehicle, & almost nothing front or rear. Correctly cophased antennas on a semi truck enhances front & rear range at the expense of range to the sides.