[EC900]

Are you charging a dead battery or was a brand new battery(s) installed? Recharging a very old (+4 yrs. old) or dead or almost dead battery won’t hold charge long and self depletes quickly. Fresh new battery charged should be at +12.8v. You could monitor the draw with a multimeter as the car sits over the course of a few hours.

Maybe the connections are dirty, not tight enough? Clean again and relight including ground straps front and rear. This is more for a no-start issue than a discharging issue, but does affect full charging.

Ignition switch might be faulty and not functioning properly, i.e. not shutting off.

Normally the key can only be removed from the ignition when in the lock or first position.
A worn key might pull from the ignition switch in the unlock or #1 position leaving all the circuits open. I take it the radio is wired to the ignition switch and not direct to battery/fuse.
Without the key in, check and see if anything will power on that normally needs the ignition switch to be on.

As the car sits (you don’t say how long) something is drawing mucho power. Glove box, trunk or interior lights could drain a battery in a couple days. For example, I left my trunk open just enough that the plunger switch didn’t disengage, my garage was well lit and I didn’t notice truck light was on - killed the battery.