depends on which one you buy..
mine was from a camping shop,
the wiring diagram came with it .......but i can explain the Principal...
a fused wire connects to the battery.... from the s/c/relay
a wire connects to the w.f.p battery... also earth the wfp baterry
now the principle.. is you take a wire to a wire in the van that is only live when you turn the ignition on..ie radio..
this means the batterys are only conected when the van is turned on ...
not draining the van battery when the van is turned off.
make sure you earth the s/c/relay as well.........
hope this helps
dave