This unit won't work with your E300. In order for this to work you will have to change out and add some electronics to your transmission. Its still the 722.6 tranny but has a bunch of added electronics, the gear that you are in is also displayed on the instrument cluster. It can be done but its a very expensive process. I have done most of the updates except the tranny, I left my cluster in Manual mode so that I can still use the cluster without error messages.

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2005 SL65
1999 E300
1995 E300
1994 SL320
1988 560SL
1987 300TD
1982 300D
1955 300SL