You didn't mention what year your 6er is?
Many people get a Bentley BMW 5-Series Manual 1982 through 1988 (green cover- don't get the later year book). Very nearly everything in the 535i is the same as the 635Csi except for the sheet metal! A lot of good info and there is a trouble shooting chapter on "Starting and Driveability' that alone is worth it. $32 to $42
Although Bentley's has some wiring diagrams, you will need a full set. I found mine at the library in a set of Michell's Manuals used by Professionals in independent garages ( Not that crappy Chilton junk).
I also have the factory manuals on micro fiche and they are terrible! There are no wiring diagrams included; they are in a seperate "Electrical Trouble Shooting Manual". These manuals are in a style popular in the '80s I call "military moron"; they are a series of little photos with statements like "Remove part 1" next to them. To make it worst, they constenly tell you to use specifications and torque values that they don't give you!! You have to look them up in a seperate book!! Which,of course, I don't have (I use the Bentley's). These manuals are available on the BMW mobil traditions CD but they are around $100. I assume that the spec, book is in there, but I don't think the wiring diagrams are. Another good place for tech Info is here:
http://home.normgrills.net/bcg/
I hope this helps some