Hello, when it fails to start is it that the engine will not crank at all, or is it that it cranks over but will not start?
Use this link to find some shops near you that can help with your concern:
www.repairpal.com/repair-shops
RepairPal Certified shops can diagnose and repair your 2006 Mercedes-Benz R350. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.