Question

i was going to put an aftermarket radio in my 2002 alero and i was told i was going to need a 130 dollar harness b/c all my security and lights and stuff of this nature was controlled through my harness of my radio but when i went to install my inline crossover to install my amp and subs the harness seemed like any other harness on any other factory radio. Do i really need that harness or were they trying to get me to buy extra stuff.