Have a 98 volvo S70. Just installed a new battery since finding a dead car, that wouldn't turn over, and no lights would come on. After one week, my daughter says the battery is completely dead again. There were no lights, etc left on, the night before. Something is drawing down the battery over night once she locks up her car. Where is the best place to start and how? What is a common fault on the S70 that might cause this? Bad ignition switch? Thanks for any and all help.

Glovebox light.
--
"Differences of opinions should be tolerated, but not if they're too different' - Sharon Craig

A glovebox or trunk light staying on is one common fault

You can put a DC ammeter in series with the battery with everything off
and expect about 18-20 DCMA That powers the CPU and the Clock

If you see higher current, you can pull fuses and hope the culprit is on a fused circuit, You'll see the current fall to a low value when you pull the right fuse.
That should lead you to the problem.

Warning: don't run the windows etc, most multimeters top out at 10Amps
and if you run a window motor you'll blow a meter fuse.

Good Luck, Bill