Best is a factory service manual (FSM) for '74 but probably pricey if you can find one. Easier because it's more common is the Haynes 620 manual which is only good for the collection of electrical diagrams and is pretty much crap for everything else. Here's one for under $13. With this and our help you should be able to sort your wiring and repair it.

https://www.ebay.com/itm/136508829481?

If you don't have a '74 FSM keep and eye out for it and get one. Pays for itself the first time you use it. Keep in bathroom and read it every morning while making room for the information. Garbage out... info in.  

got a part number from your harness? I might have one. Just not sure what year the truck is. It's a kingpin truck, so it's earlier.

pr3yanimal 0

Posted October 30

24012-B7900 is dash to engine compartment for '74 the other is the same for a '75

Dash to floor harness is 24013-B7900 (I think)

Floor harness (dash to passenger seat?) is 24320-B7900

Chassis harness under passenger seat outside and to rear is 24014-B7900

Tail harness across back is 24015-B7900

I’ll check and report back. Might be Sunday or next week as I’m headed out of town tomorrow. We’ll see!

I have a full wire harness from my 620KC - headlight to taillight. PM me if interested