Hey,
I have changed the battery and tested the alternator, so neither of those are the problem.

A couple days ago, I drove my car at night, parked it, and the next afternoon the battery was so dead that an interior light wouldn't even turn on. I made sure that I left nothing on, and got a new battery.
I didn't drive the car for a couple days, and when I went out to start it.... same thing. Dead again.

My stereo amplifier is set on a toggle switch, so no current is going to that...

Any ideas? Common problems?

Use an ammeter to measure the current draw from the battery (with the hood light removed, if so equipped), with the ignition off, and with the dome light(s) off (doors closed). The current draw should be less than 30 ma, or so. If not, you will need to track down what is drawing excess current.

is your hatch pull down working right ? sometimes when they get jammed they continue to pull power and will draw down a battery in a day or two.