Need Help finding/figuring out half dead fuse box problem

Currently I'm helping out a friend working on a 74 Challenger. I realize it isn't an A body but I'm sure the wiring is about the same on most Mopars.
The car will function just fine part of the time but out the blue it will not even turn over. The dash gauges won't register at all. The head lights are very bright also. It appears half of the fuse doesn't have any voltage. Any idea where to look???????
Thanks, Jeff