[s79bj40]

i think the consensus is that its not worth the trouble of doing it dodgy and just wiring off one battery. You need to buy or aquire through other means a converter from 24v to 12v. These are very common, i see them all the time and i dont even know many people with trucks, and they should be cheap for a second hand one. You wire it to the 24v, and 12v comes out the other side, just make sure that the one you get can handle the ampage the radio requires, which if it is a standard head unit will be lass than 15amps.

hope this helps
Sam