Can any body tell me why my balast resistor would blow, is it my coil that would be bad

 

What year is your truck? Is it a ballast resistor or a resistor wire?
Although they both have a tendancy to wear out over time. Their resistance lowers the voltage to the coil and a byproduct is heat, and so after time they tend to "burn up" so to speak.
Not a guarantee that is what has happened to yours but it would be my first bet.

Sparky