Here is what I found over on the Unique site. I used the Radiantz LEDs and they worked perfectly from the beginning. This was on a basic rolling chasis kit produced in 1992, long before LEDs were being used in cars. The trailer relay was "state of the art" at that time, whatever that means.
"Just a note regarding the use of the 48 LED bulbs made by Radiantz: You will need to use a "no load" solid state flasher with these bulbs, even if you use conventional bulbs in the front turn signals.
Edit: Most LED sellers tell you that you need a no load flasher when using LED bulbs. I've found that if you have incandescent bulbs in the front turn signals, you will still generate enough load to trigger the flasher. Always verify that your signals work when you make the change, just to be sure. So far everyone has been OK with the original flasher."
Link to the thread:
LED Taillights that work!!
Bill