no power to cab 68 GMC pickup

[welding83]

hey guys im a new member and this is my first post. i recently inherited my dads 1968 GMC pickup it has what i believe is the 250 inline 6 everything is original except for a few new parts put on this year to get it running. My dad had the truck since his early 20's took my mom on there first date brought me home from the hospital took me on my first camping trip etc. i would love to get this truck running and derivable. a few weeks agoi was heading home i started it up on a cold night once it had warmed up i pulled out the knob to turn on the headlights and the engine and every thing electrical shut off. ive replaced battery, voltage reg., and alternator im out of ideas please help!