The use of LEDs for blinkers gets a bit complicated. It can be done reliably but it takes some doing. I have had very poor results with "LED Flashers".
Let's explain how the flasher works. Current flow through the flasher generates heat. Heat causes a bimetallic strip to bend and the circuit opens. It cools rapidly with no current flow, bends back and the circuit closes. The key here is current flow.
LEDs don't draw much current. Low current flow=not enough heat to make the flasher work. LED blinkers stay on and don't blink because the flasher doesn't have enough current flow.
In some cases we can install LEDs in the rear and keep incandescent bulbs in the front, the incandescent bulbs will draw enough to make the flasher still flash. The Square Body wiring schematic will not do this, the reasons were beautifully explained by
@Turbo4whl .
The solution to this issue is made by Sylvania. A set of resistors that must be wired into the blinker circuit. They splice into the positive wire and then connect to any convenient ground allowing more current flow. Don't be confused by the resistor drawing more current, that sounds backwards BUT we now have an additional pathway for current to flow to ground. Now our flasher has enough current flow and the LEDs can blink.
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In some cases we need 2 sets of these because we need one in the front and one in the rear on each side. In vehicles made before 1968 we usually get by with only one set because before 1968 we didn't have side marker lights and the blinker circuit was more simple.
