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If you add resistance in series with the brake/turn signal bulb/led you will decrease the current to the bulb. In series, resistance adds, and more resistance means less current. Less current through the bulb means a dimmer light and a slower flash because of less current through the flasher.
If you put the resistor in parallel with the bulb you will still get full voltage across the bulb and the same amount of current through the bulb so it will be just as bright. The parallel path of current through the resistor means more current through the flasher and a faster flash.
Put the resistor in parallel with the bulb and make sure it is on the bulb side of the flasher.