Door speakers not working

I have a 2002 Dodge Ram 1500. I bought it used, and as long as i've had it the radio has been horrible. The speakers in the back door dont work at all, and the front door speakers just put out a little bass and thats it. The dashboard speakers work fine and sound as they should. I had noticed a small burn on the LCD on the radio, so i replaced it with another factory radio and no change at all. I hope someone can help me figure this out, because its driving me nuts.

Sounds like all the door speakers are shot doesn't it?

Or the amp.

I tested the speakers, and they are good. I found an amp at a junk yard for $50, gonna try it this weekend.

Yup, that's what I did for my 1992 Bravada. I still lose the right channel every now and then.