No power in cab after trying to start

[TheSeeker]

Ok so for some background my 1969 c10 has been having issues with electrical system. The issue is that when I go to start the truck it acts like there is no power to any of the accessories or lights in the cab. The dome light doesn't come on with the switch. The ignition with or without key is completely dead.

I have verified there is power from the battery to starter and the junction post on the fender. From fender block I have verified power going INTO the fuse block on the u nderhood side. Here's where things get weird.

If I go out to the truck and using my tester I can get power to the fuses and the block. If I were to put the key in or try to use the dome light or headlights etc I lose power at the fuse block and it doesn't come back.

I am not going to lie I thought it was the ignition or the headlight switch so I changed both along with the floor dimmer. No dice. Same problem.

Does this sound indicitve of a common issue I'm just not getting? Or maybe a ground issue? Any and all help appreciated. Thank you.