wires questions for CD player head unit

Ok i have bought an Alpine cda 9827 and i am installing it in a maxima 91. In the instructions od the alpine it says to use a supplied screw to connecnt the ground to the chasis.....but there is a wire for the ground on the harness i bought. Also. ont he old (but aftermaket) tapeplayer, the ground form the hraness is not used and the tapeplayer gorund wire goes to the cigarette lighter....i didnt know that the lighter could be used as a groung but anyway i want to know if i should cut the wire going to the lighter and use it or use the harness's ground wire or should i put the ground sumwhere else. I would also like to know where the usual spots for grounds are located. And i am a little confuse what the blue and blue\white wiresa are used for. i dont know if there required to be connected, what they are used for, or if they are for separate amps.