You only need one wire as the speaker frame is the second wire and is the ground.
If you are using a resistor for the voltage drop to 6-volts, that may be your problem. When the radio is first fired up it will receive a 12-volt blast which is very damaging to the radio. If you insist on operating you radio from 12-volts, purchase a device that actually renders 6-volts to the radio. This device is readily available, but a tad spendy.
If you purchase the reduction device, I know a reliable radio repair person that can restore your radio for you.
This same repair person can also convert your radio to 12-volts which will eliminate the voltage reducer and make a completely reliable radio for operation on the 12-volts.
