RepairPal Certified shops can diagnose and repair your 2009 Mercedes-Benz SL550. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
first tap the brake while you are holding the shifter and see if you feel the solenoid move.if you don't.it might be brake light switch or shifter assembly.it was common problem with earlier models.Hope this helps