1992 D250 won't turn over
 

The truck won't crank. Thought it was ignition switch. So I installed a push button start. Ran a wire on big terminal on starter and one on small through a switch. When I push button it makes one click and that's it. When I jump terminals with a screwdriver it fires right up. I only used like 18 gauge wire. Does it need to be thicker wire?

By jump the terminals do you mean the positive to the small terminal or across the starter large terminals?
If large to small then possibly too small a cable on the push button circuit.
If across the starter terminals it sounds like a dodgy solenoid