the one under the fuse box goes into the cars harness near the frame and into the cab to the hvac vacuum box. if you have a T there under the fuse box it leads aross the front bumper to the vacuum canister in the d/s front fender, take the head light out you can see it.
that line is the one marked accessory in the diagram marked in red. top of the sc to the firewall down the inner fender.