Voltage Dropper.

FOR TRUCK USE. British trucks usually have a 24 volt system, which would damage a CB radio. A voltage dropper will convert that 24volts down to the 12volts used by the radio.

As a minimum, you will require a 2Amp dropper just for the CB. If you run other 12volt devices, then you will need a higher rated model.

Do NOT be tempted to try connecting the CB radio directly to one of the batteries, as this will cause one battery to become drained (drivers have been in trouble for such tricks, and truck firms have been known to ban CBs in their trucks because of battery worries), and a bad earth or other fault could result in the voltage to the radio increasing - causing serious damage.
Also, some droppers have been known to be Switched Mode type. These will not cause damage to the CB, but may generate noise, causing interference to the radio receiver and losing weak signals.