Ok that make sense Ive seen this in older LTDs This truck is older than a 1987. more like 1979 . Check the EGR valve for leaks look at the base gasket . It probably is blowing exhaust gas directly from the cylinder head onto the coil and melting it . Just a secondary note . When replacing the gasket make sure you dont knock any of the carbon built up around round the base of the EGR into the manifold as it can end up in a cylinder and the engine will knock as if you put a rod out of it . If it does happen spray carbon cleaner into the cylinders to dissolve the carbon .Hope this helps