OOPS...didnt see the part about it needing to be a 6 volt adapter. In
that case you change from a LM7812 chip to an LM317, which is a
variable regulator. Might add 1 buck to the cost, cuz you need a
potentiometer to set the output voltage from the LM317.

You can run a 2 conductor wire off a 13.8 v. power supply and use the
'317 to lower the output to 6 volts on a line just to your Icom.

This aint rocket science, easier to do than changing a tire, and way
more interesting.