Last night I went to drive my car (03 coupe automatic) and it was completely dead. When I hooked up the jumper cables the lights and fan came on even though the key wasn't in and and light switch was in the off position. We left the cables hooked up for a half hour but the car still wouldn't start. When we disconnected the cables the lights and fan stayed on until the battery died again. I did drive through several small puddles (nothing deeper than a few inches). I think something may have gotten wet and shorted out, but I'm not sure where to start looking or how to go about diagnosing it. Anyone go through anything similar or have any ideas on how to fix this?

 

It might be the alternator.

 

So today I got the car to jump start. It runs and drives fine, but when I turned it off there's a loud buzzing coming from the battery compartment. It sounds like it's coming from the back left behind the battery (if you're looking from the front of the
car). Any ideas what this could be?

 

I used a multimeter to test for continuity. With the car off and battery disconnected there is continuity between the positive battery cable and the engine and the positive cable and the chassis. I assume that means a power wire is grounding somewhere? Does anyone have a wiring diagram of the positive wires in the engine bay or any advice on where to start looking?

 

Have you tried swapping the battery?

You should be able to find a link to a service manual on these forums somewhere...

*edit* Found the link: http://www.psedog.com/cars/fsm/g35/coupe/