I just fixed a 70 Challenger last week with this same type of problem. The other weak link in that system other than the bulkhead connector as mentioned already is the ammeter up in the dash. The main power for these cars runs through a fusable link on the starter relay , through the bulkhead connector, to the ammeter and from the otherside of the ammeter to a group of around 5 wires that are a factory soldered splice/union.
From there it feeds the non-switched 12 volt side of the fusebox, the steering colum, headlight switch , ignition switch(which when on will feed the switched 12 volt side of the fusebox and the alternator. It really doesnt feed the alternator but provides a path from it back to the starter relay and provides alternator voltage rather than battery voltage to the car when running.
Check your schematic just to be sure.
Long story short you will need to measure for voltage going in and coming out of that ammeter. The one i worked on the gauge input side burnt out, the wire was attached but the stud was just barely hanging on to the circuit board. I could wiggle it back and forth and hear it arcing. I have seen cars with bare wire at those connections from the ammeter going bad and causing so much resistance the wires were melting the insulation.
http://www.madelectrical.com/electricaltech/amp-gauges.shtml will take you to a great article with drawings.
Hope this gets you up and running.