My guess is that its a problem with the starting or charging circuit. Those are the 2 circuits with enough juice to blow that main fuse. It it were any other circuit it probably would have blown one of the smaller glass fuses first.

You can test each circuit at the fuse box the see which ones shorting though. Just clip a test light to the battery and touch the terminal for each fuse. The one that lights the test light up will be the circuit thats grounding.