So I swapped a motor into a 1980 Yamaha 400 XS. The bike has a cut wire harness in two places. One bundle is 4 multi color wired which matches to a multi wired plug coming from the engine. The other cable coming from the engine is three white wires in a 4 wire spade plug. What I think matches this is three black wires off the harness. How do I know which wire matches? Are they all grounds? Are these wires for the rectifier?