Verified answer

Could be caused by a after market stereo with a digital clock being on all of the time with about a 5 amp draw.

Is there still a drain when you pull the 50 amp fuse? Pull the fuse for the courtesy lights inside the truck, disconnect negative cable from battery and take a test light and hook it up between the negative post and cable and see if it lights up. A bad switch somewhere can cause a significant drain on the battery also. Toggle all the switches in the truck until the light reacts on the test light. Replace switch if this is the problem. Otherwise, start pulling fuses one by one till the light goes out on the test light, under the hood too. If the 50 amp fuse is the cause of the drain, find out which lights it runs and follow through with diagnosing the problem and fix if possible. Or take it to your local mechanic and see if he has better luck. Hope this helps.

The old battery drained suddenly after you put some other electrical piece on the car, like a different stereo, lights or such.

Take whatever it is you put on off and see if that helps.

Some where after the fuse the light circuit si shorted to ground.