Chevy Suburban Radio

Over time about a year ago, my speakers would cut out, and sometimes work, sometimes not. Now it has deteriorated to the point that only the dash passenger side speaker works, and that is only occasionally, when it decides to turn on. Usually, I would just hit the dash or the underside of the dash to try to make them work again, but now that hardly works. The previous owner said there is a small amp under the radio in the dash, and there are sockets for speakers under the back seats.

I ask a sound shop about replacing the wiring because i thought that was the problem. they said they don't replace the harneses, they use their own wire, and charge by the hour. But they said that its my radio thats bad, not the wiring. I have the stock radio (AM/FM Casset). I have a CD player i can put in it, but I want to know. Do you think my radio is bad or just the wiring, and if I replace the radio, is it something that i can do myself?