Due to wear, high usage vehicles can experience ignition lock and tumbler failures. The ignition lock cylinder will generally require replacement to correct this concern.Google+
Car Problem Reports
Mercedes-Benz SL320 High Usage Vehicles May Experience Ignition Lock and Tumbler Failures
Mercedes-Benz SL320 Problem
Visitor, 1997 Mercedes-Benz SL320, 111,000 mi
ignition switch-- worn down and locked-up. Had to replace via dealer and it took over 2 weeks. Check and see if you can get locksmith to do the work. I had no luck.