Steve you cannot (usually) find this with resistance checks. This is because things like connector terminals and switch contacts, or sometimes splices or wire crimps, are usually the cause, and the heat up and change under load. Also "the wiggle test" sometimes.
A good example is the ignition switch, think what you have
You have the power in CRIMP to the connector TERMINAL, the CONNECTION between the connector terminal and the switch terminal itself, then through the switch CONTACTS and out on the run terminal, and again the CONNECTION between the switch terminal and the connector terminal, and once again the wire CRIMP at the blue wire.................then the blue wire goes out through the bulkhead and once again you have CRIMP, the CONNECTION between the two bulkhead terminals.........and the CRIMP at the enging bay end on the blue
Follow this like a road map. As the wire progresses through each crimp, terminal conneciton, switch contact, etc, there is/ might be "a little or a lot" of voltage drop.
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.