Might be the switch on the column. I know its a pain to change, but if you can get the connectors off, the either jump the starter posts or if you have a spare one to ttry on there. Just a thought. I had one years ago that, because I was so broke ( how brojke was I??) I couldn't pay attention!
I actuall stripped the switch down, cleaned the junk out and polished the terminals with a ink eraser. Put it back in and worked perfectly for the years I owned it.
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72 GMC C2500 402/Th400 first truck (still have)
77 GMC C3500 CC dually Sold
70 C50 dump truck "Rusty"
87 K10 Suburban Sold
93 K2500 xcab 6.5TD
96 GMC K2500 Suburban 6.5 TD