A good friend of mine is pulling his hair out with this problem. Every couple of days when he goes to start his truck, it blows the 40 amp fuse in the power center on the fender for IGN/Run. If you manually engage the starter, the engine will still start and run, but you don't have any power for the heater blower etc.

What is causing this problem? Contacts in the starter??

 

could be but I would check for a frayed ,rubbed wire

 

has the starter ever been replaced? Mine did something similar but it was a 15 amp fuse and it turned out to be worn contacts in the solenoid causing it to pull too many amps