no output from stereo after car repair

hi i'm hoping you can help me as i've had my stereo sitting on a shelf for months now with no hope of fixing it...
it all started when i took my mercedes c180 1996 into the garage for an immobiliser issue. the mechanic seemed to not know exactly what he was doing and took 3 weeks to fix the car. when i got it back i programmed the stereo code in (they disconnected the battery a few times) and the stereo powered up fine, however i had no sound. i checked the wiring which all looked ok so i removed the stereo and tried it in my other mercedes and my husbands proton but still could get no sound....
so i've figured the problem to be with the unit itself and not any wiring but how do i fix it?
the stereo is a SONY CDXm630 it was in full working order (i had replaced the fascia ribbon a few months previously) before i took it in to the garage and i've been informed the outputs may have blown? this is a little too complex for me so any help would be greatly appreciated
have you ever had the radio tested outside of the car? there are many things that can cause a radio to have no output. you may want to check all your speaker wires and speakers for shorts and bare wires.
yes, it has been tried in 3 cars now. any other ideas?
if the output ic is blown, you are probably better off getting a new stereo. the output ic in that deck is fairly expensive. it would probably cost more to repair than the deck is worth.