xs, Party Pooper!!!!!
Where in the wiring was your friend suggesting this resistor be installed? On the ground from the battery negative to the frame/engine??? If so, I'd suggest he put his pipe away for a while...
While that wiring harness is for the XS650, it is basically correct for the XS400. Keep in mind that we have 2 ignition coils. The horn/accessory connection could also be used for turn signal, gauge, neutral and oil pressure lights