My 99 STS has recently started draining the battery completely dead after only a few days of sitting. The battery was new a few months ago. The car sits a lot so when I found the battery dead I figured I'd charge it up and go. It charged and I used the car for a day and after only a day or 2 of sitting it was dead again. Where to begin???

My first step would be to have the battery load tested.

If your battery was deep discharged it can be dead even if it's only a few month old. Since even a STS without any technical problems has a relatively high load on the battery all the time GM says after 2 weeks without engine running the battery can be drained completely. Thats why i strongly recommend using a Optima YellowTop battery in those cars (they won't get damaged when discharged too far)

To find your problem: make sure your battery is ok, before you don't know that for sure it doesn't make sense to search any electrical problems. Then read your DTCs and erase all of them (to verify to have no codes just because of your drained battery). Then drive the car a few miles and shut it off. Now you check again for DTCs and note them, it's important to know in which control module which code was set (because many codes are used in all of the modules) and if it is a "current" or "history" code, when you've done that, post your list of codes here

I also use the optima yellow top and it has been in all three STS cars I have owned and is still going strong. It is well worth the 180 I spent for it.

I can't check the codes because the dash is blacked out. I posted on another thread about a bad body control module. I still haven't fixed that so I can't check codes until I can restore the display.