I have a trailer with an internal battery to run lights, a winch, the
trailer emergency brake, that sort of thing. My previous tow vehicle had a
charging lead so that the trailer battery was charged by the vehicle while
it was being towed. The current tow vehicle doesn't have this lead
connected, so I charge the battery while stopped with a normal battery
charger.

I would like to connect up a charging lead, but I wonder if it should be
connected directly to the vehicle battery, or to a switched source? My last
truck seemed to have it direct, since I could run the trailer lights by
plugging into the truck, which would be handy.

Any ideas on how this is normally wired would be appreciated.

Cheers, Brian