Battery goes dead unless un hooked - 1998 Mitsubishi Galant

Turn everything off that you can think of. Disconnect one of the battery leads. Use an Ammeter to complete the circuit again. The current now flows through the ammeter. If it's less than 10 Amp current draw then you can use a standard multimeter in current mode (this usually means unplugging one of the multimeter leads and moving one it to a different input connector (see your multimeter manual if in doubt) This method will allow you to see how much current is being drawn when the battery supposedly has no connections turned on. Be aware that small things like clocks and radio memory are usually still drawing milli amps to keep some essential items alive. But if you have 1 or more amps being drawn continuously then the battery will gradually go flat.
Once you can see the excess current being drawn then you can disconnect various fuses to determine which is the offending circuit and narrow it down to a device.
If you have no multimeter at hand then buy a cheap $10-$20 meter. They have a multitude of uses in a vehicle or around the home. You can substitute a 12v lamp in place of an ammeter. Not very accurate but the more current that flows through the lamp the brighter it will be.
Good Luck

You lights all. One can be on and the bbulb maybe burn . Something is draine it. Stereo. Something is on and you don't know-it .