All of them require in-line relays, even if you make good arrays or use high powered LEDs because the current an LED draws is so much lower than that of an incandescent bulb. An incandescent 3157 draws 21 watts for the bright IIRC which equates to almost a 2 amp draw for each bulb. Our trucks use thermal flashers, which basically rely on the current being drawn through the flasher to heat an element which opens and closes the circuit as it heats and cools. This is what causes the flash. The in-line resistor makes it so that your circuit is drawing a greater current so that your flasher will still operate as normal, since even a 5 watt draw is less than an amp of current.
Like I said before, if you want to do it the PnP route, get them from Elef-USA. They are spendy, and sometimes the wait time is a little long, but they are the best option aside from making your own array or paying someone to make an array.
