My vote is for the...
...license plate light as well. My lamp-out warning light kept coming on as well. All the lights checked out OK and it was driving me crazy, until I looked at the reflection on the rear chrome bumper. It was only then that I saw that one of the license plate lights was out.
1989 300TE "Alice"
1990 300CE "Sam Spade"
1991 300CE "Beowulf" RIP (06.1991 - 10.10.2007)
1998 E320 "Orson"
2002 C320 Wagon "Molly Fox"
Res non semper sunt quae esse videntur
Not in this weather!