have you considered just adding an external resistance to the 6v selenoid coil…this will drop the voltage down to 6 volts
if you can measure the current draw of the coil while activated at 6 volts you could estimate the operating resistance under load.
V/current= resistance
adding an equal resistance value ( try one good for a couple of watts) in series with the control wire will drop the 12 V in half and you r coil should work fine…without burning up.
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