My truck battery keeps dying after 2/3 days, why?

Based on your story, I suspect a couple of things: First, if that is the original battery in the truck, it's 4 years old and may be failing. The expected life of a battery doesn't go much past 4 -5 years. Stop into one of those automotive stores in your neighborhood and ask them to "Load Test" your battery. The next thing I would look at would be the alternator. Is it recharging the battery back to full capacity? That can also be tested pretty easily, and it should be putting out approx 14 volts. A "quickie" test you can do yourself (be very careful) is to touch the center of the back of the alternator with a screwdriver. You should feel a magnetic pull. The magnetism will tell you that the alternator is putting out SOME voltage. oh and "Duh" make sure there is water in the battery and that your alternator belt is tight enough to turn the alternator pulley continuously. If the battery, alternator, and belt test good, then there is a problem with the installation of the stereo or a fault in the electrical system.

One thing to consider, Do you park it in a garage? Is it a metal garage? if so, is the garage grounded to earth ground. some garages can form an electrical field that will kill batteries, just went through it with mine. grounded the garage to true earth ground, no more dead batteries. Or, could be answer # 1

guaranteed it's something that you had installed and it's not hooked up right causing the problem.

12 Amps is a lot. Average light bulb will draw 1-2 amps if that, so it probably isn't door switch or glove box light. Start pulling fuses out one by one while the ammeter is hooked up to narrow it down. When the amps drop you will know which circuit your draw is on and can start looking for the problem. Make sure you take into account the dome light if you are working on it with the door open, or remove the bulb. If your truck has a security module it will draw amperage until the module sees the truck is secure (all doors and windows closed), then it will go to sleep. On cars that have one (I didn't think Silverado's did.) the module will stay awake and draw current from the battery if it thinks a door or the hood is left open. All I got.

Did they "Load Test" the Alternator? to see if it's putting out the Listed "Max." Amperage. "Alarm System pulling 12 Amps? Wired Wrong, with the Truck Off and the Alarm not sounding it should pull no more then 1 Amp to keep the System active. "Sounds like they (Who Ever) are just "Guessing" since it's your money they are Spending! Take it to a Good Auto-Electric Shop or the Dealer to get it done right.