Got mine from local 'Trailer Supply' company £14 an auto Sparky friend fitted it for me 5 years ago, never had any probs whatsoever, been great!
Two heavy duty cables from main battery, earth to van, job done, I use a sealed battery for WFP which came off a golf Trolley, been fitted now for three years never ever gone Flat, never removed, just works... as things should.
Good Luck