1976 Other Lincoln Models Ignition Switch

I believe I'm having a problem with the ignition switch in my Mark IV. I've traced the wires back through the SSI and the car will crank but won't start. Using a Chilton's manual I've tried all the tests and even was able to jump the wires from the harness to the ignition but the car still wouldn't start. How do I find and replace the ignition switch? I know it's somewhere near the firewall at the steering column. Thanks in advance for your help.