A friend of mine was having some problems getting his car to start, so we replaced the battery. Unbeknownst to us, the previous owners toyed with the wiring on the terminals, making them the same colour and we crosswired the battery. We hooked it back up properly and it behaved as if the PCM was destroyed (no power to anything but the headlights and a couple of gauge indicators), so we replaced the PCM. No change.

The "Main" fuse under the hood (the 100-amp fuse in the engine compartment fuse box) won't keep. Shorts the very instant the battery is connected.

What's the most likely culprit? I don't want my friend to have to buy a new car if it's something simple.