I have this same problem

I had a mtx 250w amp in my car a month back with 1 10, worked fine, now the other day i got a kenwood 400 off a friend and tried to hook it up to the same 10 inch sub the amp light would turn on but no sound went to the sub, so seeing how it worked before the same wiring set up i said well this amp must be blown up no big deal. So my friend says hey lets put my system in your car. exile amp and 2 12inch subs and still we have the same issue so i try everything i can think of change the plugs the rca's are in nothing, we try new rca wires nothing, so i think maybe my decks screwed ( had one that was going in soon anyways ) so i replaced the deck but still we have the same issue NEED HELP!!!