When I got the car... the battery soon crapped out on me. So I bought a new battery from Canadian Tire.
Under warranty and everything.
But soon after it dies...
I dont play my subs, unless my car is running.
The only thing that stays on when my car is off, is a lil blue led at my rear view mirror.
I checked the altenator.. and its putting out power...
Although it kinda, SLIGHTLY goes down after a while.
New Altenator? 200$

replace then

do you have a voltage meter?

what you need to do it measure the battery power before you shut the car off at night. The next morning measure the battery again. If there is a big difference in volts. (drop below 12.60volts ) you have a parasitic drain on your car.

second issue if that does not seem to be the problem would to check your cables and ensure there is no oxidation.