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Same thing can be accomplished with a capacitor assuming that the flasher is a RC Time Constant circuit. I would be more concerned with the ohms vs the wattage of the resistor. You could probably get away with 25W but no lower due to heat, 6ohms is baseline for the resistance.
All Resistors with the exception of Potentiometers or variable resistors have no polarity, they can't be installed backwards either way works the same.