My first thought if your alternator burnt your wires is to look at your voltage regulator, is it good and is it grounded. Or are you running the magnum alt controlled by the 5.2 computer. 1975 is a bit new for me, does the truck still use an ammeter. Regardless if you are using newer more powerful alts, you should run the alt output directly to the battery, not through the original harness the wasn't designed for a bigger alt.
You say you got a refurbished harness. Was it done right for your year?? Looking at a 75 wire diagram online it looks like you need to supply power to the orange wire at the ignition switch connector. This wire then needs to mate with an orange wire in the engine compartment at the bulk head connector. This orange wire goes to the starter relay.
I'm assuming your starter relay works and is wired properly.
I would disconnect the orange wire from the relay and use a volt/ohm meter to see if you have continuity between it and the orange wire at the ignition switch connector.
You say you got a refurbished harness. Was it done right for your year?? Looking at a 75 wire diagram online it looks like you need to supply power to the orange wire at the ignition switch connector. This wire then needs to mate with an orange wire in the engine compartment at the bulk head connector. This orange wire goes to the starter relay.
I'm assuming your starter relay works and is wired properly.
I would disconnect the orange wire from the relay and use a volt/ohm meter to see if you have continuity between it and the orange wire at the ignition switch connector.
