Please Help with 12V Power

The positive wire, trailer brake wires, and trailer wires (that hook to the tow vehicle) come through the front of the frame then up into the trailer in the front underneath the bench seat of the kitchen table. There are many wires spliced and capped together in this area. Then a stream of wires go to the front of the trailer to the left around all the appliances back to the area underneath the couch. All the circuit breakers and power inverter are in this area. The 120AC pigtail also comes in on this side and hooks to the breaker box. What makes no sense to me is when I have all the wires outside hooked up as they were to the battery and I run separate wires to the power box itself it will just spark as it is narking but when I unhook those wires coming into the trailer and run single wires to the power box the lights come on and no sparks or narking.