As pioneer4x4 said, check for battery drain.

1. remove the negative terminal.
2. Hook up an ammeter/multimeter between the negative cable and the negative battery post.
3. you should see somewhere between 15 and 30 Milliamps.
4. If you see considerably more, start pulling fuses one at a time in the under hood fuse panel until you isolate the culprit.

Once you find it, report back.