Now I don`t have a 87 Truck I have a 86 and A 88 here.
The fuse you pulled unless your wiring has really been modified is the Hazard/Horn Fuse!!
The only down side is these not working with the fuse pulled.
Which really makes me think there is a alarm or parts of one in the truck.
Did your Horn ever work??
Did the Hazard lights (4 way flashers) work at all ??
Will either work with the fuse pulled?? They should not but at this point who can say.