Steve,
On most of the SMC era Beavers, there was no speaker for the dash radio. However, the dash radio was wired into the Bose system to use it as a speaker system, therefore since the Bose system is powered by 120 volt AC current, the inverter must be on to listen to the dash radio while driving. To change this situation, all that you have to do is wire an 8 ohm automotive speaker to your dash radio, and it will be just like your car, no inverter needed.
Gerald