I owned a 99 Regal with the Monsoon system. It sounded awesome for a stock setup. One of my rear speakers blew. Sounded like shizzle if you get my drift. I decided to go ahead and upgrade.
Got on
Crutchfield and ordered a set of Infinity 6x9's, nice ones! Freakin excited as hell, I jumped the gun and ripped out the stock rears to make room for the new stuff. The speakers arrived and they looked beautiful!! Installed them in a jiffy and to my surprise they sounded like SHIT //content.invisioncic.com/y282845/emoticons/crap.gif.7f4dd41e3e9b23fbd170a1ee6f65cecc.gif
I got on the phone and proceeded to speak with a rep about my problem. Connections were good, speakers were good. Monsoon amp = bad! The Infinity's didn't match the amp. Monsoon = 8 ohm, Infinitys = 4 ohm. At low volumes they sounded nice but as soon as you cranked the volume, they would bottom out and had horrible output (not to mention no bass)
You have a couple of choices.
1. Stay stock. This is my recommendation. Go to
http://www.gmpartsdirect.com and put in these numbers:
10328736
10328737
Those are the replacement parts for the rear speakers. GM recommends you replace them together. For around 60 dollars you can replace the rears.
2. Go after market
I dont know much about that part of it so maybe someone will chime in to help on that part.
PM me if you have any other questions.
Good Luck!