Test the battery and alternator.
Engine off, lights on for one minute. Battery voltage should be 12 to 12.5 across the terminals.
Engine running, alternator voltage should be 14 to 14.5 acoss the battery terminals.
If it checks out okay, check all wiring from battery to alternator or battery to ground.
In addition, read this thread.
http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=trouble&Number=1043318&page=&view=&sb=5&o=&fpart=all&vc=1 
