When I made my rear bar I used LED lamps and did not have any issues, but when I changed the indicators in the bullbar from incandescent to LED I got the dreaded super fast blinking.
I guess that by adding the front indicators I went below the resistance that the ECU would accept. To fix the problem I simply used commercial LED load resistors that were wired in parallel with each indicator's power and earth wire.
As mentioned earlier, fitting an electronic relay is the best solution as using load resistors negates any power saving benefit of fitting LED lights but, if like me you fitted LED lights because they are normally more reliable, then it is not as much of an issue.
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I should add that on some of the newer trucks (the FGB-71 as an example) it can be a real pain to replace incandescent bulbs with LEDs. Just ask the guys at ATW how much fun they have had doing this. From what I was told, there were some circuits where it was considered "just too hard". Whatcharterboat would be able to tell you all about that.