Did you hardwire the head unit or use a harness?
At any rate, if it powers up but you dont get any sound, it is because the factory amp is not turning on.
Like killfox said, check the lead feeding your oem amp. It should be a similar colored wire (blue/white or solid blue). IF you make sure this lead is properly connected and you still don't get sound, try the power antenna lead. It's one of the two, either should work....