Hi, Done the '12S' socket thing on our TD4 for pikey wagon.
Looked at the LR kit but went another way.
Fitted a PRC4427 (LR voltage sensing relay driver from Ebay around £5), a HD relay, some fuses and a few lengths of 2.5mm wiring front to back inside a 10mm flexi-conduit under the freeby alongside the path the fuel lines take to the rear nearside of the towbar.
Fitted the power feed from the spare 6mm hole on the battery clamp, in-line fuses as in attached diagram, bolted PRC and relay onto spare studs on suspension turret under ECU. Used plenty of sleeving over exposed wires (flexi-conduit, Ebay again, right up to relay) and conduit right up to 12S. Plenty of cable ties to hold it up and no interfering with the wiring inside motor.
Pull the one fuse and all power is cut.
Gives a good 12 volts at the socket and cuts the 'charging' feeds when the voltage at the battery falls below 13.5/14 volts.
Picked up the reversing, brake light and fog feeds at the rear and fitted relays so as not to load the cars wiring and fed power to relays from the AUX feed from split charge to trailer. Any faults on trailer wiring (except indicators) will not kill car circuits only trailer.
Worked for me.