my battery is fully charged but my battery light is on and my ultinator is fine what would cause this

how did you come to this determination? More details make for a more detailed answer

We purchased a new alternator and still the same thing. Battery light keeps coming on . We made sure the battery is fully charged. Even had a garage test the alternator and said it was fine as well as the battery. If it is the cable between the two how do we change it or even know if thats the problem? Is there fuses involved or any computer system?

there may be a fuseable link between the alternator and the battery. you can easily check to see if it is this wire with a basic multimeter and check continuity across the wire. My reccomendation is to check all connections are tight, check that the voltage rises when the car is started. not running it should be just over 12V when running it should be around 14. if this is the case, Iwould think the light is in error.