98 cadillac deville battery wont hold a charge

Sounds like a 12V drain on the system, like a trunk or glove box light that won't shut off.> Could be many. many other reasons why. The way I would find the draining circuit would be to remove the neg cable of the battery. I then would connect a 12V test light between the neg battery TERMINAL and the neg battery CABLE. With a 12V drain the light will shine. I would install into the light a 9V battery powered recepticle to hold computer and radio station memory. Then I remove fuses 1 at a time, then reinstall, at the same time looking for the 12V light to go out. Hope this helps.