How can I get my factory AM/FM radio to work?

[kna4977]

When I got my truck, the radio that was in it was a factory AM radio. It would light up, but there was no sound. I had to take it out to work on the A/C, and when I put it back in, I was unable to plug the radio connector back in correctly, and therefore it didn't light up like before. I bought a factory AM/FM radio and installed it. I did a circuit test with a test light, and there is current at the fusebox and at the red wire that connects to the back of the radio. However, it does not light up, and there is no sound whatsoever coming from the speakers. Also, when I turn the radio on with the switch in acc or on and the interior lights are on, the lights dim, which makes me think the radio is trying to work. Any ideas on how I can begin to tackle this issue?