please elaborate on "Nothing else electrical functioned"...this may be your avenue to explore. relays typically last a long time unless subject to water/moisture & act the way you described when not enough current is passing through them. make sure all grounds are in great shape & use di-electric grease wherever possible...you know check battery ground to cab, frame, engine etc...in the extreme poor grounding situation the clutch cable will become the ground (for a very "short" time) & will snap.