Not sure on an 1989 Truck but on a 1989 E-series in the wiring harness about a foot or so from the firewall connector in the engine compartment there is a group of four fuse links sticking out of the wiring harness. One of them is a large yellow wire.
I have not seen this on a 1988 van so the 1989 truck may have these fuse links also.

A Photo of them:
http://i54.photobucket.com/albums/g9...onFuseLink.jpg