I'll give it a shot....
The first picture shows it routed over the muffler, to a clip on the upper frame rail, then forward under the toe board into the engine compartment.
Inside the engine compartment it just loops back through the firewall.
Once inside the cab there is a d-nut near the firewall hole for another clip, clip is missing in my picture but the nut is there.
Not a good under dash picture but it routes through the hole in the channel under the gas tank to the speedometer.
That extra wire is for the water temp gauge.
My truck was a farm truck....it once had a hot water heater...that is what that patched hole was for...