Connection to P38 lighter socket in Boot

I have a solar panel to mount in the P38 Boot. I plan to use the lighter socket which will have to be converted to be permanently live. I have read the various threads on this same subject which recommend the feed direct from the battery (+) terminal or connect to a Becm (+) screw terminal.

Is it possible (as an option) to connect into a feed from one of the trailer socket feeds either the 12 or 12S wire bundles. If so which cable (if any) is permanently live when the ignition is switched off. I've checked the socket outlets with a multi meter and all pins seem to be dead when the ignition is switched off...maybe there is a problem with the wiring!!

Better to go direct to the battery with a fuse at the battery end.