Try this first:
Here are a few things to try. One, remove the negative cable from the terminal and put a test light between the cable and the battery post. If the light glows, there is a drain. To find what circuit is causing the drain, remove one fuse at a time until the light goes away. Then, install them one at a time. When the light glows there is a drain, when the light is off, the bad circuit was removed. You may not get the light to turn off. In that case, it means that there is a drain in some other circuit that does not run through the fuse panel. First place I would look at is the starter. Starters have been known to short out at the solenoid and cause a constant drain to ground.
Long Live My Bonneville 97 SSEi Dark Cherry