As I understand a regular flasher won't work...not enough current (not voltage) flows through and the bi-metallic strip doesn't get hot enough to operate...at least not properly. I would just buy the led flasher. It is completely different (as are the led bulbs) and will work correctly with them. I would also not combine incandescent and led bulbs in the same circuit.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
I'm just about to order some LED tail lights. Both my back lights were burnt out and the flasher still worked just fine with front lights only. I don't think my flasher is a LED flasher; probably a heavy duty one. Anyway, I'm sure that the LED lights will be OK with this flasher as it works with or without a load.
In shopping for LED lights, I see some are rated for 12 volts and some are rated for more than 12 volts (e.g. 18 volts). The ones rated for more than 12 volts are a bit more expensive, but I think it's a good investment since the alternator can put out up to 15 volts. The burn out problem was mentioned by some reviewers.
I have had good results using a standard bimetal flasher if I only replace the front or the back with LEDs. If you want to replace both, just get an electronic flasher for $5.
I dislike the resistors because they increase the current draw and dump all of that extra energy into heat.
The lights come in 3 colours; white, red, amber. Did you use red or white bulbs for the rear brake/turn/running lights? The light fixture has a red lens. White bulb may be too white even with a red lens. Red bulbs may be too dark with the red lens.
I put custom LED housings in the rear that have running, brake/turn, and backup lights in one. I can get pics later.
In the front, I experimented with amber and white with both amber and clear lenses. I decided that I liked the amber lenses better cosmetically and both white and amber bulbs looked basically the same behind them.
The white bulbs behind the colored lenses are brighter than the colored bulbs behind them because the LEDs are single wavelength and the color does not quite match the lens color. That results in more of the light being absorbed by the lens.
The white does wash out the lens color a hair, but I couldn't tell the difference unless I was looking for it.
If you get good quality bulbs, either the colored or white ones will be plenty bright behind your colored lenses.