'91 Bonneville ignition switch
 

How do you replace 'Ignition key Cylinder' 1991' Pontiac Bonneville SSE?

once you get the lower dash and steering column cover removed you can see the lock and tumbler. If you put the key in and turn it to "on", with the battery disconected of course, there should be a button to push on the side of the exposed cylinder and thus the cylinder or lock and tumbler will slide right out of the column. If all else fails, alldata.com for 24 bucks a month will give you an amazing guide with diagrams and detailed pics for any yrar make or model, its really worth it!