mustang killing the battery

i have a 2000 GT mustang...it is draining the battery..it has a new battery new alternator and cables..the car runs with the cables off..the battery dies if left connected overnight. had alternator tested and it is fine. car was fine and this issue happened over night...help.:banghead:

Havent had this problem with my mustang but my first car was a dodge avenger and I did the same repairs and had the same result. I ended up spending about 600$ before i found that one of the cables was stripped and was grounding out by touching metal in the engine bay. I fixed it with electrical tape. I guess what i'm trying to say is just make sure the cables are properly installed and make sure they werent stripped during installation. hope this helps...