In this period of production, EMD put a sample electrical schematic inside the back of the Operator's Manual, so if you can find an intact SW8 Operators Manual, that should have the proper schematic. My recollection is that this locomotive model and the SW9 were covered by a combined manual (#2313?).
If you cannot locate a suitable manual, another alternative is to call Rail Systems Inc. in Houma, Louisiana and ask for the Service Manager. He may have a suitable drawing on file or be able to order one from EMD.
"We Repair No Locomotive Before Its Time"