BEM-S4

Dumb questions. I want to rewire a car charger to have a RJ45 instead of the plug that goes into the cigarette lighter so I can plug into the RJ45 in armrest. Had bluetooth kit installed so the DB25 connections have been made already. I know I need to wire the power to the switched power pin and the ground to the ground pin.

Here's where the dumb questions start. Should I just cut the plug off the charger and wire to RJ45? Do I need some kind of in-line fuse? How do I tell which is hot and which is ground?

Any info would be greatly appreciated. Thanks!