Battery wont hold a charge

If you can find an ampere gauge hook it up and pull one fuse at a time.
When the amp draw goes away that will be the circuit drawing the battery.
Then trace the wires from that circuit. Could be as simple as a trunk light on. But the amp draw test will show you where to look.

If you have had the battery in the vehicle for more that 2 or 3 years, then they can have what they call a dead cell, and they can often test good. Go to the part store and ask for a alternator tester to make sure its good. If the alternator is good then check all power connections.

If all fails then you have a problem with something shorting out and draining the battery

There is something drawing amps(voltage)from the battery.It can be anything,from a clock,in the radio,to an amp,or a light on in the trunk,or under a hood.