WallyP

While there are LOTS of possible causes, here are a few common things that you should check:
- Do the interior lights work as designed? That is, do the interior lights come on when either door or the hatch is opened, and go off either as soon as the door/hatch is closed (early cars) or a few seconds later (later cars)? If not, the problem may well be that the little red lights in the edges of the doors are always on...
- Glove box light off when the door is closed?
- Under-hood light off when the hood is closed?
- Is the alternator belt REALLY tight? The flat ribbed belt should be really, really tight. The V-belt should be really tight.
- Are the battery terminals really clean? A visual inspection is NOT adequate! A battery post can appear to be perfectly clean, and still have a high-resistance layer. Use a knife or post cleaner - if the post is clean, the lead cuts like butter. If it has a "crunchy" feel as you cut it, it is dirty and must be cleaned.

These won't solve all of the problems - just almost all of them.

antsmands

Here, Here, cleaned my main ground and my dead battery and low voltage reading problems went away!

123quattro

My rear wiper seized in the not parked position. It was always on trying to get there and drawing a couple amps. That will drain a battery in a hurry (and is kind of difficult to figure out).