Sounds to me like a bad starter solenoid or related wiring. The battery in these is at the back and cables run all the way up to the motor. trace them through and check for rubbing/shorting. If that is all good, find the starter solenoid. The usual test for those is short the terminals with a large screwdriver briefly to check that it cranks. If it does but not on the key it is likely that it will need replacing.
Those wires up front are for a different engine, they use the same loom between models and some don't use all plugs.
