My 1999 Cadillac DeVille is losing power even though everything is turned off on the inside?

I am trying to determine what the cause of losing power on my 1999 Cadillac DeVille seems to be. I bought it used and used it for a few months with no problem and then my brother uses it and doesn't remember what he pushed, and now it seems to lose power when it isn't being driven. At first the engine wouldn't start and seemed to lock up for some reason, but we got it working and now it has trouble keeping power, even though the battery is relatively new.My 1999 Cadillac DeVille is losing power even though everything is turned off on the inside?the altenator would by my first guess. if its not out completely it may not be generating enough of a charge to re-fill the battery and run the power of the car.My 1999 Cadillac DeVille is losing power even though everything is turned off on the inside?You have a drain somewhere. Unplug your negative battery terminal and use a test light to jump from the cable to the terminal. If the light is on, that means you have a drain somewhere. Now, easiest way to find that drain is to eliminate the circuit it could be in. With someone holding the test light, go to your fuse panel and pull out a fuse and see if the light goes off. If not, put that fuse back and go on to the next one. When you find the one that makes the light go off, then you will have a general idea where your problem lies. :)