it almost sounds like you might not have a good connection on your speakers. Are you positive that the connections are solid? Did you solder them? Are they butt-end connectors? Are they twisted together and then a cap put on them? Did you utilize the factory speaker wire from the head unit to the speaker? Do you have a factory amp in this car? what kind of car is it?
You say that previously you didn't have any speakers connected to this head unit...is that to say that you simply didn't have any speakers in your car at all or that they were going through an amp previously?