86 cougar starting problem

I have a 1986, V6 3.8L mercury cougar. I just changed out the battery and the alternator. I also changed the serpentine belt and the oil. She ran great, but then after i let her sit for about 2 days the lights didn't come on when i opened the door and no power to start her up.

i have the feeling that something is draining the battery out while it is inoperative, but i have no idea what it might be. i even unplugged the trunk light because i had a feeling that that might be a common problem. i had the battery tested and it came back as a "good battery", but i'm thinking it might not be able to hold a good charge. i'm not sure what is going on with it.

If anyone has a similar story, or problem that they've been able to fix, let me know, i am completely baffled as to what could be draining the battery.

here's a quick update on my situation...I tried charging the battery via jumpstart, and the battery cannot maintain a charge at all. i spent a good 20 minutes jumping it and to no avail. I am starting to think that it could be a bad battery after all...:banghead: (even though the shmuck at the large supercenter told me otherwise) so, now i'm in a pickle...replacing a battery that is less than 3 weeks old or....? i am uncertain to why the battery would just die without being used other than the oldschool analog clock that is constantly running...i am almost certain it's a bad battery...BUT, if you have something to add or to subtract from my dilemma, feel free to comment.