The converter is a great solution and probably the best idea. However, a simpler way is to just connect the radio to one of the excavator's two batteries. 24V heavy equipment uses two 12V (or sometimes four 6V) batteries in series. If you tap off of the first battery, you get 12V. This results in uneven load between the two batteries, thus slightly overcharging the other battery - but if the load is small (like a stock car stereo) then this will not cause any problems. Be sure to use an appropriately sized fuse placed as close to the battery terminal as possible.