I am buying a 91 5.0 LX tomorrow that the current owner hooked the battery up to backwards and fried something. Anybody know what it might be?

 

Probably the computer.

 

When I saw this, I was thinking about a 6 volt battery. You hook them up backwards from a 12 volt. I know it don't help you.

 

could be the alternator or alternator cable..
could be many things. Try to start the car and see what happens.

 

It probably just blew a bunch of fuses. I'd start there.

 

I hope it's just fuses, but fuses are on the positive side of a circuit before it gets to the loads or switches and power flows in that direction towards ground. So if you put a positive charge on the negitive cable, in theory the power flows backwards throught the loads and swtches first before it reaches the fuses. So cross your fingers and start at the beginning. The good news is that you may have only fried a fusible link which is inexpensive.