The standard sound systems in these cars are pretty notorious for being bad quality. I had the same problem as you and when i took the door panels off the rubber around the speaker cones was completely dry rotted, same for the ones in the rear deck. I went to a local junk yard and picked up a set of bose speakers for around 20 bucks and swapped them in. Its still not the greatest system ever but sounds much better and was cheap to do. Of course there are other routes you could take but this is probably the easiest and cheapest.