Audio Systems: crackling noise in speakers when stereo is on, jeep grand cherokee laredo, jeep grand cherokee

Question
I hear a crackling and sometimes a low buzz sound on the right rear and front speakers when I turn the radio on.  It happens more on AM than FM, if that is important I don't know.  Do you have any idea what it is?  I have a 1994 Jeep Grand Cherokee Laredo with the stock cassette/radio that came with the car.  Do I need to replace the speakers? amplifier?  Can I install a radio/CD player to replace the one I currently have?  I saw several on Ebay that are very reasonably priced, but I didn't want to buy it if it doesn't fit.

I know I've asked more than one question, but I really need some help!

Thank you for your time and I eagerly await your response.

Answer
Time for a new radio. if you have the infinity system stock, there is an amp under the rear seat, but most times it is pretty reliable. Most failures happen in the fader/balance control on the factory cassette deck. You can pick a nice replacement CD player, and with a dodge/jeep fit kit, and a adapter harness, it's a pretty quick install.
This link should help;
http://www.installdr.com/InstallDocs/Jeep/PDF/533017.pdf