86.5 supra electrical short need help!

A friend just gave me a 1986.5 supra non turbo. its been sitting for a couple of years and now i have it . i tried to put a battery in and found out the Pos battery cable is a dead short to the NEG battery cable. i have isolated the starter from the POS cable and that wasn't it. the whole car is dead and lifeless. does any one have any suggestions as to what the problem is?
Thanks