03 GMC 2500 battery problem

[donthekawguy]

Just picked up a RCLB 4x4 3/4 ton GMC with 105k on the clock. Drove it around for a couple days and everything was fine. We took it out to dinner and when we got home I started turning the headlight switch to see what the auto selection does. Nothing changed so I left the switch in auto. When I shut the truck off I pulled out the key and when I opened the door the dinger came on like I left the key in the ignition. I shut the door and the dinging stopped. I blew it off and went in. When I came out the battery was dead. I hooked up the cables and it fired right up. I let it run for a few minutes. I went out a few hours later and it just clicked like the battery had a little juice left. The ignition switch is a little loose and the headlight switch moves a little bit because its loose. I also was messing with the button on the switch so now I'll have to go thru and figure out what I did wrong. lol Any idea where to start? I did drive it to work tonight and it will sit for 12 hours before I try to start it so I'll see whats up in the morning. I'm thinking a bad ignition switch, bad battery or parasitic draw.