There should be several larger looking wires that have a vinyl/plastic looking wrap over them, up near the battery cable. The fusible link can open without much more than a barely swollen looking section on the vinyl/plastic surface. Here's a link to a great forum for Ford trucks. Go to the main forum and scroll down to the "1968 to Present Full Size Van" or use the appropriate F150/250/350 section and ask the same question there. There is also an Electrical/Wiring forum further down for more specific information. Good luck!
www.ford-trucks.com 
