you may already have it in the back of the radio

the original radio may have an adv setting which raises the volume the faster you go, so make sure one of the original loom wires for the radio doesn't already have it in there

on the granadas it was black and on some of the later ones i've seen it's yellow assuming it's got one, but they are on different plugs

a pic of the back of the stereo and the loom plug that is originally on the car should help