I took my 99 ford escort zx2 up to autozone to replace the battery. Bought the battery and the guy was installing it for me while I was on the phone. The guy put the battery in backwards and blew my 100amp main fuse. I replaced my starter, which turned out to be bad but wasn't the issue, replaced the fuse and as soon as I connected the positive cable and attempted to connect the negative cable, it started to spark and blew out the fuse again. Anyone have any idea on what it could be?

 

You have a short to ground somewhere. A nice one to pop a 100amp instently. There are a couple of different ways to chase these down. First do a very good visual inspection. You're likely to find your positive cable has been pinched, or worn threw somewhere and touching metal. Good luck with it.