That PDF shows a lot of clips that I doubt you will find on a Mustang antenna wire. That's good since the fewer clips the more likely you can fish out the old wire and fish in the new one using the string method they suggest. I also don't know that you can disconnect the antenna wire from the antenna mast on an original type antenna so I don't know that the string method will work.
If the string method fails its fairly easy to remove the mud shield from inside the passenger's fender. Its only got something like 5 small bolts holding it in there one of which is under the hood near the hood spring. With the shield out you can then reach up in there and do anything you want with the antenna or its cable. Taking the fender off is definitely NOT required although that would give you maximum access to get the job done.
