There are sites and products that we are happy to support - we only list those that we have direct experience with.
Rob Siegel has written a book to help the enthusiast walk their way through their car's electrical systems - something near and dear to our hearts! Don't be afraid of this book - it's written (almost) like a novel, and it's written at a level that pretty much anyone can relate to. It contains so many wiring 'basics' that I think it's a benefit to owners of all vehicles, but it's a must for BMW, Audi, Mercedes, MINI, Porsche, and VW owners.
The Hack Mechanic Guide to European Automotive Electrical Systems
If you need electrical parts for your BMW, Ducati, or Moto Guzzi bike, I can't recommend these guys enough.