My 1988 560SL has the same problem. The Electrical diagram shows the shift pattern illuminator, the radio illumination, and the intstruments Illumination Rheostat on that circuit. Oops, there's also the Front Cigar Lighter with Ashtray Illumination.

Anyway, I was unaware that the ashtray was supposed to be illuminated, mine isn't. The cigar lighter works, my instruments are lighted, but my shift pattern illuminator is not working even with the new bulb (the old bulb still looks good). The wires look good all the way back to where it goes between the glove box and behind the radio.

BTW, I think the shift pattern bulb is 12V and 2W. I took my old one into a specialty bulb place and they had them in stock for about $1.25. The local Pep Boys did not carry them.

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Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino