radio/heater core problem

I finally changed out my heater core and it works so nicely and i'm so happy to have heat now. Everything worked after I put it all back together EXCEPT the sound on my radio!!:banghead: The radio works just fine and it looks like its playing a cd or the radio is tuned in, but i just have no sound at all. I took it to best buy since they had installed the radio a couple of years ago and it's still under warranty but they couldn't figure it out. I figured it has to be something behind the dash, not just behind the radio where they looked. anyone have any ideas about a connection i could've forgot about??

forgot to add, it is a 96 chevy blazer 4.3L 2WD!