Battery goes dead?

Something is draining my battery, causing my car to kill. I went through all the normal channels, including changing the alternator, battery and terminals. It still died. I next came to the terminal cables. I was thinking there was some corrosion on the positive cable coming from the alternator, so, I bypassed this by using a 10 gauge wire coming straight from the alternator to the battery. When I hooked up a fluke meter, it read between 12-12.5 volts. When I turned on my lights, it dropped to 10, when I turned on the ac, it died.

I'm not sure what else to do.

Thanks.

When I shut off the car it will drain the battery. I connected a volt meter between the negative cable and the battery post and pulled fuses until the voltage went to zero. The fuse that I pulled was the 100 amp fuse at the fuse box next to the battery positive terminal. Any ideas what this would feed? Discharging of the battery is the only symptom. I remove the fuse evertime I park the vehicle for more than a few hours and it will restart OK. Thanks.

The brake and battery lights are on. Replaced 4 year old battery. The new day and a half-old battery was completely drained this morning. Car started with a jump-start and runs like usual. What is causing the lights to be on and what is draining the battery? Thank you.