It's been my experience that when the Diagnostic Socket doesn't light up, the OVP relay needs to be replaced. The good news is that they are cheap so cheap that you should order two new ones at the same time. I have had new ones fail right out of the box. This is highly unlikely with two and if one happens to fail, send it back but you'll still be able to solve your problem.

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03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio