I'd recommend a good manual, the original LR publications are available from
Brooklands Books, but for quite a bit of stuff on Landies the Haynes manuals are adequate, including the wiring diagrams. Haynes manuals are available at Halfords, and quite possibly high street bookshops.
As the diagram is part of a copyright publication the is no website, to my knowledge, where it is
legally available.
Regards
Wolfie