Adaptors won't work on a Road King (at least none that I could find!)
Gotta get cutting and splicing those wires to get it to work.
It's a pretty easy job, took me about 20 minutes.

The one thing you will need is this:
https://www.harley-davidson.com/stor...wiring-harness

This connects to the headlight and you splice your harnesses into it.
This is the wiring diagram. You want the HDI model.
https://serviceinfo.harley-davidson....6?locale=en_US

Hope this helps!

 

Pretty sure all you need is an H4 to H9/H11 adaptor.

Custom Dynamics CDTBH42H9H11 LED Headlamp Adapter for 2014-2017 Harley Road King Models

 

Eagle Lights has an adapter that worked great on their led for me looks great cheaper and plug and play

 

Be very careful!

This only works with the Custom Dynamics LED light. It doesn't work with the JW Speaker/Daymaker.

Believe me I know......

 

Again - it may well work with their OWN lights but beware if buying for another manufacturers light!

 

thanks for all the advice, guys, really helpful. The adaptor I've ordered is the Eagle Lights one mentioned above, clearly it's being sent via donkey as I've been waiting ages lol.
The LED I have is a Wismec one I found on amazon, used it for a while on my old Dyna. If the adaptor ever shows up and doesn't work I'll use the Harley adaptor Plasmadaddy used although I may have given up doing it cheaply by then and give Harley more money for a real one!
One of the guys behind the counter in my local dealers warned me not to use a non Harley headlight as there is a risk my bike will catch fire...😲

 

Finally got my adaptor today. Holy crap these things are bright.

 

I had to cut my harness and solder an adapter for my Kuryakyn headlight. Not bad but not something I wanted to do this early. When I install my bars I will go back in and add connectors to make it look cleaner and make it easier for removal or switching.

 

I like it! The red is pretty unique.

 

Hopefully the LEO's in your state won't harass you for the red halo... certainly illegal here.