Adding a amplifier

Hi all,
So I want to add a amplifier, my car is the non hk sound system so it don’t have a standard amplifier in the boot/trunk, where is the best place to get a signal for the converter??
behind the radio?? If so does anyone know what colour cables do what? I will need 2 pairs of cables so probably best if they were the cables for the rear speakers.
Failing that where else is a good place?
any help would be greatly appreciated
Tia