Mod: Landing a new power Cable

[mkmorris188]

I'm currently in the process of doing the stereo upgrade for a 2013 Nissan Rogue SV with SL Package. I have run all the wire through the doors, car, etc, new #14 to the door panels, have all wires landed except the power wire. I can't seem to get onto the power lug at the battery with the #4AWG Cable I have. I was wondering if anyone had any tips on how to latch onto there, or if they know if a big enough lug that I could use to terminate underneath the existing connection. Any feedback you may provide is greatly appreciated. This is to power a 5 Channel Amp for even distribution through the car.

The biggest issue I face is I can't seem to get the nut that tightens the power cable down off and its just sliding. And the stub for the power lug is ginormous. So I'm open to either or another option.