Need help stereo will not produce sound

Ok so I have a 99 sr5 and I had installed a while ago a sony xplode radio. Well shortly after having the radio installed it got to where it would not put out any volume. I had to turn it up all the way to give it any volume at all and then I could barley hear it when I was driving. Well then I figured out that if I scooted the drivers side seat all the way back that it would fix the sound like there was a short under the seat and then when I moved the seat back up it went back to low sound. Well shortly after that the radio went dead completely no lights no power nothing at all and I've drove around without a radio for forever and now I've decided I want it fixed. Well the guy I took it to fixed it to where it turns on and works fine but it still has the same problem of having very low volume even when turned all the way up. I know its not the speakers themselves because I had him install new ones. Please help me figure out whats wrong.

P.S. I took the seat out and could not find a wire under it.

 

Sounds like a bad head unit.