check the cables all the way to the starter. so you can find out where you get your power that goes to the key,some equip.will have the supply on the same terminal at the starter some split off at the battery, if it splits off at the battery make sure that you don't have an inline fuse,you might have to just keep tracinf the cable from the battery feed to the switch, and if necessary probe into the cable along the way. I assume that you have tested for power at the switch with a light oor meter,not by just turning the key. I have had cases of having good voltage at the battery but when a load was applied the voltage would dissapear,this was caused by a bad plate in the battery, did you try jumpers ?