This car ('72 280SE 4.5) will run beautifully when it feels like it. The problem is some days when I get into it and put the key in I get absolutely nothing! No ignition lights,no power to the windows, zilch! I thought it could be the ignition switch, but I get no power to the headlights either. Unless the ignition switch and headlights are related in some way. I'm stumped, because some days it will crank right on and run perfect. It's not the battery because I had that tested. Any ideas? Appreciate!Thanks!

Sounds like the classic dirty cable ends on the battery.
They look good, but are not connecting.
Take them off and clean them up.
Also , check ground straps.

Been There...
 

I have had the same problem once in a truck, and it was the battery connections, they looked clean but were not giving me any power, once I clean them no more problems. I would check, ground cable also..