Well the easiest route to go is take it back to circuit city and ask them whats wrong, and tell them to fix it. Hard to say exactly what the problem is, but I know a lot of people who replace the stock head unit without replacing the stock amp, so that probably not the problem, however it could be. For starters though I would ask/tell the people at circuit city to fix it. If they refuse, just pull a "can I see your manager" because if you paid for installation, the thing should work for more than a week.