Connected a light pen from to neg cable to neg post to observe when light would go out when fuses were taken out.

Discovered the removal of 9 would make the light go out, but ..

only when C and 9 we both out at the same time.

If 9 was taken out, but C was left in, the light would stay on.

9 controls the diagnostic module, among many other functions.
(Started with http://www.peachparts.com/shopforum/tech-help/366497-check-engine-light-after-key-out.html)

Noticed the transmission selector lever position light is not working - controlled by 9 also!

What does this mean and what should I do next?

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1991 300E 124.030 103.983KE 722358 03 412178
207K

1979 240D 123.123 250K (Project car)

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