I have a 92 400E as well (mine doesnt have the button and light, I believe it was only required in CA).

Anyway, the 38 pin connector is under the cover of the computer box. The box where a battery would normally be - there should be pne round cover about 2 in in dia. Use the appropriately sized socket to remove (its a 1/4 turn like a gas cap) and there's the 38 pin in all its glory. Alternatly you can remove the entire cover - 4 allen head bolts.

I think the cover you are refering to is the connector for the electronic throtle actuator. Its about the right size and is in that area.

You will need to build a code reader to get the code pulses from the 38 pin socket. This is covered in a variety of places (do a search) including my favorite
http://pages.prodigy.net/jforgione/MB_S500.html

He is a member and posts regularly.

Paul