I have a '99 that I put a pretty sweet system into. How much are you looking to change? I don't know if the speaker sizes changed, but I know im my car I have 6 1/2" speakers in the back (one on each side), and I believe it's a 5 1/2" that's in the door down by our feet.
Here's what I did:
I replaced the 4-previous mentioned speakers with some Boston Acoustics (god I love how they sound. my absolute favorites).
About a year ago, I took out the back seat, and put this in instead:
That's a 500w JL Audio amp, and those sub's are 12" (Boston Acoustics ProSeries). Needless to say it sounds amazing.
The sky's almost the limit if you aren't afraid to pay more for custom work. But you will notice a tremendous difference just getting the factory speakers out of the doors.
I would love to put a system in my truck but I have a rergular cab and there's really not a lot I can do to it. I am 6'6" and need all the space I can get so putting subs behind the seat is kind of out of the question. So the Boston Acoustic are good replacements? I had thought about Kicker or Kenwood since I had had them in a previous ride and was happy with them.
Sorry for the delayed response,
Hey, if you were happy with the Kenwood's in your old install, go for 'em man! Boston's are of exceptional quality, but a lot of it is what sounds good to your ears. Boston's also are fairly expensive. Good luck and let us know what you go with!
Hey also wanted to mention I love Kicker speakers all around, but if you want some bass get som JL 6.5" subs yes thats right Kenwood makes some also but I've heard the JL's before and liked them, then have a small box made to fit behind the pass seat, it will make all the difference in the world, some nice components up front if you want to spend the money and she'll sound super sweet, don't neglect on a good head unit with high and low pass crossovers too, it makes the speakers work where they were designed too.
If you got questions, I got answers!
In my 04, it takes 6 1/2" all around, but you can always check with the salesperson just to be sure. I also went with Boston Acoustics front and back and installed myself to make sure it was done right. The factory speakers are probably closer to 7 1/4, and might even accept an 8' in the back. The rear speakers even have a crude tuned port in the cab sheet metal that makes a noticeable difference. I also installed an older Alpine 360w rms(4x90/3/2) with a Sony head unit. I put the amp under the driver's seat by cutting away the carpet and thick foam and am building boxes for a couple of RF 8" subs that can be removed for space considerations. Very nice sound and still have most of my cab space.
Not sure what a good inexpensive speaker to recommend is. Infinities used to be good, Polk makes some nice stuff. Go to a electronic department store and listen to what they have, and then check on ebay. My Bostons list for $260 a pair and I found them on ebay for $150.