The MAD Electrical website has you cutting and splicing the factory wiring. You can read it here.
http://www.madelectrical.com/electricaltech/amp-gauges.shtml

I'm not doing it exactly that way on my 68 Dart. I'm just going to keep the factory wiring intact but add the extra bypass wire. The bypass wire has a very low likelihood of shorting to ground but I'm still going to add a fusible like to it for my own piece of mind.

I'll be using relays on the headlights and on the convertible top motor to take as much load off the bulkhead as possible. I already bypassed the AMP gauge and will run a volt gauge instead.