Am not a chav.... but I have a cup spoiler so was thinking :s

I've bought a speaker to RCA adapter mine has 5 cables from it (left +ve & -ve, right +ve & -ve, ground)

i am thinking I of cutting and soldering it into the head unit hardness. Does this seem okay? Can I use the ground on the harness for the ground on the converter?

also can I use the ignition to the head unit for a 12v remote on the amp?