Cat 314d Electrical Short

I just aquired a 2011 314d and have no knowledge of previous owner or history. When I bought it the batteries were dead and it needed a boost to start. Barely had the juice to start the next time so I charged both batteries. Now when I connect the cable it sparks. I turned the battery disconnect switch off and there is a bigger spark when connecting the battery. Pulled out fuses one at a time and tried to connect the battery but same result. Someone hooked a wire to the positive side and one to negative side which goes to a box mounted in the battery compartment so I disconnected that but still sparks. There is an aftermarket emergency shutdown switch and another switch which looks like aftermarket that says cold weather start. Any ideas what to look for? Thanks

Cat 314d Electrical Short