It seems you have a direct short, but you already knew that.

When I run into something like this I just take to an automotive electrical shop and pay them $20 to figure it out. I might not have them fix it, but it is worth it to have them troubleshoot it just because of the time it saves.

The 'starting with a jump' sounds like a dead cell in the battery. Perhaps you could have it tested? I have bought them brand new and found them to have a dead cell just due to a problem from the factory.