Date registered: Jan 2006
Vehicle: 73 280SE 4.5, 71 300SEL, 03 BMW Z4, 72 Fiat 850, 80 Triumph TR7, 85 Porsche 911
Location: DC Area
Mentioned: 1 Post(s)
Quoted: 95 Post(s)
So it drove home and overnight, the battery died? Take the battery to Autozone, Napa, etc and get it checked. If possible (and I'm assuming you can get the car running), have them check the charging system too.
Assuming both the alternator and battery are good, you have a slow discharge through something. Aftermarket radios and alarm systems are usual suspects. To further refine what the issue is, turn the ignition off, and using a multimeter and on the current/amps setting, remove one fuse at a time and place one lead on each of that fuse's terminal. The current will go through the multimeter and give you an indication of what's drawing juice.
Have you seen indications of melted wiring, or charred remains of the same? Can you smell burnt wiring?