STEREO ETC STAYS ON EVEN AFTER TURNING IT OFF AND REMOVING THE KEY

That means that you have a parasitic drain, that can be caused by a short or by a defective device.

First try to understand if there is a parasitic drain.

Once you exclude the drain, replace the battery, a battery with shorted cells can cause the drain.

To measure drainage remove ignition key, connect the amp meter between the battery positive wire connector and positive terminal.A couple of milliamps drainage is normal, and is caused by instruments, car stereo memory, ECU etc.
Start removing more fuses , and open the wiring at different points , trying to locate the problem.
Remove the fuses and see if you have drain by connecting a meter to read amps on battery, if you have drain with no fuse, then the problem is insulated to wiring harness before the fuse, with something shorted between the battery and the fuseblock.
Also check car stereo and car amp HU, the HU can keep a unit on when the ignition key is off, causing drainage.