The switches are not listed individually.
You may be able to substitute another '3 position' switch - although I know of none on the E320 cabriolet. Or rebuild your existing switch using parts raided from a regular (e.g., window) switch.
But first carefully check the circuit boards (I think there are two). Mine had a couple of bad solder joints that when fixed, solved my courtesy delay and switch malfunction issues.
You probably already know to be careful handling the plastic cowling. All the plastic on these cars gets brittle with age. Mercedes charge well over $400 for a replacement courtesy lamp assembly.
Good luck.