Chrysler AM/FM/CD

In most factory systems, just swapping out a standard head unit with a factory premium head unit would not even work, because the factory premium head unit would require the factory amplifier to power the speakers.

Fortunately for you, your infinity system is designed to use a high level--albeit low power--signal from the head unit to input into the factory amplifier.  This means that the infinity head unit itself can tecnically drive 4 speakers on its own, though obviously it will not have anywhere near the power of a complete infinity factory system.

Did you use an infinity deck from that same model year? (such as an oem part from a dealer)  Or did you take it out of a newer vehicle (i.e. 01 or later)?  Not only would this affect how it is connected to the vehicle's wiring, but it could also affect how it would perform in your vehicle.  Also, did it work properly in whatever vehicle it was taken out of (if applicable)?

Check all the wiring and make sure the deck has a good ground connection.  I would say your problem most likely has something to do with having a bad ground.  Unless of course the unit itself needs to be repaired.