Some more detailed troubleshooting may help. What wiring diagrams are you using? The voltage drop is either across the ignition switch contact used or a component loading the circuit down.
Is your cruise control working? The same 12 volts from the ignition switch contact used feeds at least three other fuses. Cruise (10 amp) and HVAC (10 amp) for sure. So check the cruise control operation.
Normally with a bad ignition switch you get other symptoms or other accessories not working properly.