money saver

ok, i was told upon installing a head unit (cd player) that if my truck was an 03 or newer that i would have to buy a 100 dollar adapter cable...that is not the case however i own an 02. Now upon visiting another store i was told that i would need that same part if it was an 01 or newer...i proceeded in just buying the wiring harness need for a g.m 94-05 model vehicle and figgured i would just do it with out..can someone please tell me if i have to have this seemingly unessciary part. my thinking is that the bodies didnt change or the models didnt change hardly at all until the 03 why would they make them the same from 96-00 but then have to but a new part from 01-11..can someone help me please?

the connectors may be the same but the door chimes and audible alerts play through the speakers via the bcm starting around the 2000 model year. This is why you need the adapter.