Ok an update and it gets odder. CC port only kicks out about 5v so that's great for LEDs, not much else.
1. When the Speedo switched live is plumbed into the switched live on the bep unit, neutral light doesn't work. Everything else fine.
2. When the Speedo live is plumbed into the permanent live, all the N and gear lights work fine. But only when the killswitch is set to OFF!
This doesn't make sense to me. What can possibly be causing an issue from the killswitch against the permanent feed? It could be a feed into the BEP that's coming from the OEM lunchbox plug?
So essentially, I reckon it's the killswitch being ON that's killing whatever connection is required by the BEP unit to display gears and neutral. I can't fathom it!