03 387 Peterbilt, CM 870 electrical nightmare crank no start

[RocketScott]

Slapping an unknown fuse box and wiring harness into your truck is almost guaranteed to be a headache. Wiring requires attention to detail

You need to get everything connected correctly before trying to troubleshoot. All the little electrons need to go where they are supposed to and stay away from where they're not. Ignoring that will keep it from running at best. At worst you'll let the magic smoke out of components