CB coming over the AM/FM is usually caused by either a front end overload of the AM/FM radio or the antennas are coupling.
Reroute all of the wires to and from the CB and maybe move the antenna.
Step 1) Ground the chassis of the CB, with a separate wire or ground braid, from a screw on the rear of the CB chassis to a good metal point. Check the grounding point with a multimeter to make sure you have a good connection to the frame.
Step 2) Re-wire the CB with both the red and blk wires going directly to the battery with a fuse on the red wire as close to the battery as feasible.
Step 3) Check the antenna mount for grounding to the frame using the multimeter.
Step 4) If steps 1,2 and 3 don’t do it, temporarily mount the antenna in a different location and see if the coupling of the antennas is still a problem.
NOTE: Maybe if you can borrow a mag mount antenna and mount it at the rear of the vehicle and see if that helps and then try it out on the roof and see how that works or vice-versa.
Maybe consider the temporary antenna moving first, remember it is all theory nothing more, what will work on one vehicle wil not on another.
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