Hi
I'm new to here. I usually do as jyeager mentioned. cut then solder and shrink tube them. If you decide to cut the harness then leave about 1.5" of wire on the harness just in case you ever wanna put it back ( I wouldnt but you might). Here are common Nissan colors.
Speaker(s)
RF(+):BROWN
RF(-):BRN/WHI
LF(+):BLU/WHI
LF(-):BLU/YEL
RR(+):BLUE
RR(-):PINK
LR(+):RED
LR(-):GREEN