If you measure say, 12V backprobing the power into the ignition switch, and the blue coming out shows say, 11.8, then you know right there in your hand is a .4V drop. It is right there in those 2 wire ends, the crimps, the connections and or the switch.
One way to check the switch is to get some good quality individual "push on" flag/ blade terminals that fit the switch TIGHT and crimp / solder them to some scrap no 14 wire. Clean the switch terminals really good, nice and bright. "Rig" this up with a load, to a battery in fact you can clip lead it right into the car where it came from, and then clip your meter across the power in and the blue. Now you have pretty tight connections at the switch. If you measure any drop across the switch........and wiggle the key........see if it changes, now you know it is right in that switch itself.
