Yes, that is what diodes do, but it's not obvious to me how you're suggesting you use one to accomplish your goal. Bill is an EE, so perhaps he will chime in here. Otherwise, perhaps you can draw a schematic.
An easy way to determine which of those wires leads to the radio is to pull all the fuses to the DC circuits except the radio. According to the wiring diagram in the TM blue binder, the radio is the only appliance on that circuit. Turn the radio on, and turn the volume up to a decent listening level.....might as well enjoy some tunes while you're working. Using a clamp-on DC ammeter, check each of the wires back there for current. The only wires that should have current going through them are the ones that supply the radio.
Sears sells such a meter (which is also a multimeter) for about $50, which is a small fraction of what all the others ones I've seen out there that measure
DC current. It also comes with a thermocouple, which is cool, although I haven't used it yet. Thanks to Wayne / harveyrv for pointing it out to me.
Dave