Using a 6vdc to 12vdc converter would be a lot easier than rigging up a switching device to disconnect the charging system from 6v duties and connect to a 12v regulated load, even if the 6v generator would do the job. As long as the 12v radio didn't require more than 5 amps or so there are a lot of parts to do this job for under $50. Google "6v to 12v converter".
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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