This US-made isolating trailer wiring harness has components developed specifically for Indian motorcycles to power trailer lights, protect the bike's computer and avoid fault codes. Compatible models include the Roadmaster, Chief, Chieftain, Springfield, etc.
This kit works with trailers that have separate brake lights and turn signals. If you're not sure how your trailer's lights are set up, check out this video that explains the difference between the two systems.
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Indian motorcycle wiring harness
It looks very well made.