No moonroof means you shouldn't have the 6-speaker amplified setup. It was only offered in a package along with the moonroof ('03-'04 Moon & Tunes package or later became Sun & Sound and added a sub for '05-'08.) The amp would be mounted on a bracket underneath the passenger seat. The additional two speakers are the tweeters at the top of the door panels behind the mirrors.
Here is a thread with RPO codes you could reference.
So I'd hazard a guess that your install just got a little smoother with no pesky level adapter thingy to wire in to tone down the white noise from the factory amp.