If you ground out each wire separately at the sensor, either the Hot or the Cold lamp will illuminate. If neither lamp lights, you have a wiring issue. If only the hot lamp lights, check the bulb for the cold lamp. Another way to check the sensor, only the cold side, is to use an ohm meter, check to see if you have 0 ohms at one of the terminals to ground. If the sensor was installed into the manifold with Teflon tape or too much thread sealant, the sensor won't ground out.
I hope I made sense with the answer. My 63 GP had the cold lamp problem as you. Mine was the sensor.
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