Got to agree with Trilux, you must get the aerial connections right first.
Without getting into the technicalities of impedance, matching or loading coils … yet, check the cable with an ohm meter (a multimeter on resistance).
Disconnect the aerial plug from the rig and check the following:
Outer shell to vehicle ground or ground plane – low resistance
Centre pin to the aerial – low resistance.
Outer shell to centre pin – high resistance
The power wiring should have both positive and negative wires, and no part of the aerial in the power chain, preferably taken back to the battery but seeing as it’s just a 5 watt CB rig and not a high power police mobile radio, it’s not really important where you take it from.
The aerial cable must be 50 ohm coax (uniradio UR76 or similar) and not TV coax.
Any grub screws used for adjusting the length of the aerial must bear down tightly onto clean metal; check for varnish or paint and remove any in the area. The same applies to the connection of the cable at the base of the aerial mount.
You say that the aerial is mounted on the wing, so that should provide a reasonable basic ground plane, although the size and shape might have some effect on the VSWR as well as on the directivity, but we won’t worry about that yet.
You could also check the cable between the rig and the SWR bridge.
I too used to be a “radio doctor” and a licenced radio amateur (GW3YSP).