condition of the battery? & charging system? & cables/ belt?
parasitic load test? (google that)
PS: i answered a similar ? to another member; he apparently didn't check the basics & found 2 causes to his problem
http://www.automotiveforums.com/vbul...d.php?t=950824
checking alternator output via free on-vehicle testing at your local parts store (autozone/ advance auto parts) would rule that part out
checking the grounds/ belt tension/ and parasitic load testing is up to you...