Kenwood HS-5 headphones convert to stereo.
Modern radios often have onboard audio enhancing effects that require stereo headphones. The popular Kenwood HS-5 headphones are usually supplied wired for mono in the plug and in the shell.
Gently prise off shell assembly on the side of cable entry. Locate two red wires soldered to tag. Best not to desolder them but rather cut them off close but not touching the tag. (If you try to desolder you maybe damage the fine coil winding.)
Then solder the two red wires together with insulation. Connect end of cable to a stereo plug with TRS: white to tip, red to ring, black to sleeve. Check continuity of your work and when you're satisfied all is well reassemble the shell assembly. Job done.
With regard to installing attenuators in the headphones to reduce residual background hiss from some radios I would prefer to do that in the plug (you can get large body plugs) or make up an external patch lead with two 330 Ohm approximate resistors in series with the red and white wires. That would be less invasive to the headphones.
73 Bob G4PVB