mine was done using the accessory power point in back of truck (cigarette lighter fitting) not sure if fitted to all models , this would then give you a switched live , nd no need to take trim and carpets up.
or the split charger on this page which i have use as well and cheaper
http://www.western-towing.co.uk/acatalog/Towbar_Electrical_Accessories.html
I've used western towing for caravan brake shoes and cables in the past, excellent service!
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