You don't say which vehicle you are referring to, but if you're sure of the connections, try unplugging 1 speaker at a time and attaching a known sound source (transistor radio eg.) or an AA battery (listen for a 'pop' from the speaker) across the leads, to verify that all speakers work. If they do, then it's the radio, or the connections to it. The antenna should go up with 12V applied to the trigger wire, probably blue or blue/white. You need it up to get a signal, but the radio should put static through the speakers at even low volume.

__________________
1986 560SL
2002 Toyota Camry
1993 Lexus