So investigations as suggested by the team - following the cable from the plug to the connectors - revealed the brown wire on one side is fully severed. Not sure how the tail lights came on briefly, but maybe the final complete separation only happened when I unwrapped the electrical tape the installer used. I presume crushing by the chain in the Voyager design caused the breakage - I can't think of any other way it'd be severed. 

So hopefully splicing the brown wire back together should fix the issue. I understand a join mechanism and heat shrink sleeve is the preferred way to do the join for best waterproofing. Fingers crossed, I'll be ready for my warrant next week!

I will wrap the whole thing with lots of electrical tape, and I've crammed some strips of hard foam under the security chain to lift it and try to stop it crushing/pinching the light cables in future. Voyager having a hole in the side of the main trailer beam for the electrical cable to come out of - rather than being in contact with the chain - would be a much more sensible design.

Highly unlikely in the trailer or truck plug as one side works. 

Unlikely to be the LED as both side light and rear light would have to be the same intermittent fault. 

Follow the wires back from the plug and expose the joint. 

Thanks very much for that input, I've started to unravel the electrical tape somebody - presumably the installer of this set of lights (the second ones) - wrapped around the outside of the wiring, so I can find the joint area.

The Voyager trailer seems to have a design issue (to my eyes) in that the security chain comes out of the same place as the wiring cord, and if you get into a tight backing angle (sideways or over a dip where a beach ramp meets sand) the chain can basically jam the cord against the steel front edge of the trailer bar the split trailer lights cord comes out of. 

I think the electrical tape was put around the cord by installer to try to give it some protection.

Good thought. I don't have another trailer, but can drive to a mate's on the weekend and do that check. 

I've also sprayed some Inox on both parts of the connection, and scraped around with a very small screwdriver - which an electrician mate suggested in case there's a bit of crap on something. I think he said Inox is a conductor, as opposed to CRC. Not sure why.