Resistors, sometimes called equalizers in this application are typically needed when upgrading from incandescent turn-signal bulbs to LED turn-signal bulbs, which then the resistor is placed in parallel to the LED bulb to serve as a dummy load to the vehicles turn signal circuit. If the vehicle is already designed for use with an LED bulb, then this resistor is not needed.
On either the driver or passenger side, there is typically one turn-signal bulb at both the front and rear of the vehicle. Vehicles that use incandescent turn-signal bulbs utilize a circuit, or electronic flasher module where the timing of the blink is dependent on the load of both incandescent bulbs functioning, therefore when one bulb burns out the blink will become twice as fast to let the driver know they have a burnt-out bulb. Since automotive LED turn-signal bulbs present a different type of load to the cars blinker circuit as compared to incandescent, the blink timing will be rapidly fast without the use of the dummy load resistor when upgrading from incandescent to LED.
Most modern day blinker bulbs usually have three wires going to them- one ground, and two positive wires. One of the positive wires is for the tail light filament and the other positive wire is for the brighter blinker/flasher filament. The resistor would be wired across the ground and the positive blinker/flasher wire in a parallel configuration.
A resistor is needed for each LED blinker bulb. A common resistor value for most vehicles is 6 ohm @ 50W such like A137168-ND. If the blinker is extra bright and uses two bulbs per blinker housing (less common) then a common resistor value for most vehicles of this type would be 3 ohm @ 100W such like A103830-ND.
Your nearest auto parts store may have something more user friendly with wire leads already attached instead of solder lugs, or they may also have an LED compatible electronic flasher module to simply plug into the flasher socket so no external resistors would be needed when upgrading to LEDs, but these are just a few options to consider. If you have further questions on this topic, please reply to this post.