Factory turn indicator flashers are designed to flash quickly if a bulb is burnt out. Unfortunately, the low power consumption of an LED looks like a burnt-out bulb to these flashers, which leads to "hyperflash". Hyperflash isn't cool.
You can address the problem by adding resistors, or you can replace the flasher. Replacing the flasher is the right way to do it, but it's a little more expensive. Resistors can get hot, and they do burn a little more energy. Okay, maybe you're not that worried about the latter. But if you want your car to think it had LED turn signals from the factory, a new flasher is the way to do it.
For the NB1 (1999-00), we have an aftermarket flasher that has to be spliced in. We include everything you need, including zip ties, to clean everything up and make it all pretty.