electrical system problems

if anybody cannot understand what i am about to explain, please leave me directions on how to post pictures. there have been two issues. i recently just replaced my wiring harness on an 1980 cb750F. i started from teh back and worked my way up. everything was goign just fin until i reached teh headlight. this one cable plugged into where the headlight was supposed to go in on the old wire harness. so i just cut it off, but everything else was matching perfectly. i cut it off at a single blue wire that was connecting it to the main harness. was this wrong? did i get the wrong harness? this extra part had colored wires that didn't appear anywhere else on the old harness. and another thing. there is a whitish thing under the seat that is a part of the elctrical system what is that, is it essential for ignition? same thing for the rectifier, is it essential to just hear teh bike start? and is there a way to test teh starter motor with a multimeter to see if it works without taking it apart. i know there is alot here so thank you very very much for your time.