My son’s 1999 Saturn 3dr coupe’s battery drains when it sit undriven for a few days. We replaced the battery, but the same thing happens. Where could the source be?
If you want to find the source of the battery drain, remove the negative battery terninal. Connect either a twelve volt test light or a meter between the negative cable and a ground source. If the light lights up or the meter shows voltage, there’s a drain somewhere.
Now start pulling fuses one at a time. If the light goes out, or if the meter shows very little voltage, that’s the ciruit that’s drawing the current. You then have to trace that ciruit to find out why it’s drawing current.
Tester
Thank you, sounds like a great start. 
The first thing I would do is check to see if some hidden light (glove compartment light, underhood light,etc.) is staying on when it isn’t supposed to be on. My dad had a 1973 Oldsmobile Cutlass S that he bought used and had the same problem. His mechanic couldn’t find it. I went down one evening and started looking around. I happened to glance at the center console while I was under the dashboard debating which fuse to pull first. There was a bead of light. Removing this bulb solved his problem. If you don’t find it this way, use Tester’s approach.
Re using a voltmeter: A GOOD meter, ie high impedance, will almost always show significant voltage under these conditions, as it will need only a microamp of leakage to show 12 volts. Use a test light.
I had a similar problem with a jetta, but it was intermittant. Ie, had a dead battery every 6 months or so. By trial and error determined it was the dome light – sometimes it stayed on. If the car was parked unused for several days, then it killed the battery. Solved by turning the dome light off.