then bear in mind the TV antennas are built into the rear bumper

It would be worth checking that both antennas and both TV receivers are working properly.

Unplug both antennas from the video module (TV tuner)

Then connect each antenna, one at a time, to the first antenna socket.

Tune into a weak analogue station and note picture quality.

Then try both antennas on the 2nd antenna socket.

At the end compare notes: you should have tried 4 x combinations.

With a car standing still you can expect about the same reception levels from each antenna and from each tuner.

If one tuner is showing poor reception, replace the video module.
If one antenna is showing poor reception, check the antenna connections to the bumper antennas. This may mean removing the bumper and cleaning all connection points.