How does a 76 C10 get its main power feed

[pound41]

Hello
My Dad's old truck 1976 c10 stepside 454. When I turn the key I get nothing.
The headlights don't come on with a good battery. I have several 71/72 Chevy blazers. They have a power lead coming off the positive cable that mounts to the junction box on the fender. From there it feeds the power to the truck. That would be where I would start if I had an issue like this with that truck. I see a junction box on the firewall of my dads truck. But there is not feed coming directly from the positive cable. So how do I start to trouble shoot and chase the problem. Any help is greatly appreciated.