[ant7]

As others [Alan] have stated, you need to go through the process of elimination methodically, otherwise you risk chasing your tail, first you need to jump the starter high current side of the solenoid to see if the starter will turn without the need of the yellow wire being activated by the ignition switch, if the starter turns, then the issue is with the yellow wire, or the source of power to the yellow wire, you can check this by putting your DC multi-meter positive probe on the yellow wire connection on the solenoid, and your negative to the engine, and get someone to turn the key while measuring the voltage at the yellow wire, if when the key is turned to the start position the voltage at the yellow wire dips below 10v, then you need to check the source of the power going to the ignition switch, hence my suggestion in an earlier post of checking the fuses and their connections!
Hope this helps.
Ant.