I do make gizmos on occasion and a small , say 3A, voltage regulator is not difficult to put together.
The unit would be encapsulated/water proof and have an input lead for 24-30 vdc and an output lead for 12-14 vdc plus a ground connection.
The case would be the size of a couple of cigarette packs.
One of these units could be installed for each 12v-device/radio. Typically a CB radio needs about 2 Amps max.
Your only consideration would be heat dissipation, the box will have to be mounted so that it can dissipate the same amount of heat as the device it's powering is using.
This is a linear type of voltage regulator, which is very "clean" and will not create any electrical noise.
If there is any interest in this, let me know. Initial cost $25.00 including shipping within CONUS (until I find out how much money I'm losing).