Voltage reducer problem

[Smokey046]

Attempting to install a 48v reducer in my Club Car DS. My reducer has a positive and negative input, positive and negative output and a “key” wire. The input + wire has a 15 amp fuse in line.

I installed the input + to the first battery in series. I connected the output + to the fuse block positive stud. I connected the output - to the fuse block negative stud. I connected the key wire to the key switch. Now, when I connect the input - to either the last battery negative stud or the B1 stud on controller, there is a huge spark and it blows the in line input + fuse every time.

I attached a CRUDE quick illustration of the wiring setup. I have tested all connections with meter to no luck. I did have a couple of switch’s off of the fuse block (driving lights and light bar) grounded to the frame up front. I read a few moments ago that is ill advised. Surely that isn’t causing the short?

I am thinking faulty reducer? Any advice is appreciated!