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The system should work by just hooking the + lead to the - terminal on the car. LEDs are polarity sensitive but not filament bulbs. The flasher shouldn't be polarity sensitive either. It has a bimetal strip in it with contacts. As the current flows through it it gets hot and warps breaking the contact. It then cools and makes contact again - making the lights flash. LEDs do not draw enough current to heat the strip which is why you need to use an electronic flasher with LEDs.
You won't even need to wire it through the ignition switch since it will only draw current when the stalk is moved for R or L turn.