Here is a great source of "strange" bulbs. Obviously, you will have to dismantle your radio and check the number on the bad bulb, but you'll have to do that anyway.
Good luck!

Miniature Replacement Light Bulbs-Miniature Light Bulbs-Numbered Lamps

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'76 240D-Sold
'78 240D-Sold
'85 300 SD, 165K-Sold
'88 300 TE, 165K-Sold
'64 Porsche 356C Cabriolet- under restoration
'86 560SL 124K Miles-Sold
'94 320E Wagon, 74K Miles-128K Miles JUNKED
'06 E350 Wagon, 84K Miles
07 SL550, 14K Miles