This unit is located under the passenger seat on our LHD 1991 Hymer Camp, but has nothing to do with the base unit, only the habitation. No 1 lead, (pictured) runs direct to the hab battery positive. N0 2 lead runs off into the main loom, and is not an add-on. No 3 & 4 leads run also off into the original habitation side loom. It is obviously a relay, but it's rated at 35 Amps (at 12 volts)! The main habitation supply cable, coloured red, also leads from the battery, so this unit is not for the main supply to the rear. It feeds SOMETHING, as wires 4 & 5 must be the switching side, but what??? If it was an add-on, then the cables would be separate from the main loom bundle, so therefore it must have been fitted from new, (maybe by customer request?) I have no idea at all......Possibly an old supply to a 12 volt telly???? Any ideas most welcome. Dave.