No start...starter or switch?

[Crowe]

My 300SD let me down the other day. It would not start. The battery is good and fully charged. It appears to me that either the switch/key or the starter is bad. When you turn the key to glow the plugs the light works normally but then when your then turn the key to the start position nothing happens. No clicking or sound at all. I had to have the car towed back to the house. How do I diagnosis the problem?