I am currently working on replacing my sound system in my 94 E420. I also replaced the system, if it can be called that, on my 88 300TE.
Here is what I have discovered. The cutouts on the dash are 4,5/16" and really permit nothing larger than a 4" speaker. In almost every case, an adapter must be fabricated to install dash speakers.
E class cars since 1992, I believe have amplified door speakers, at least, as standard. 400E might have a standard premium system. On a 1994, this means, amplified (1 way) units in dash and doors, with 3 way units on the rear shelf. This appears to be an interesting sound system design, since in almost all cases, detailed sound staging should be created in front of the listener and not behind !
Anhow, I intend to replace the dash units and leave the door/ rear units for the moment.
My recommendation for the moment would be to obtain a wiring diagram for your car from your friendly MB dealer.
The easist alternative thereafter is to replace the dash units with some high quality coaxials. Good brands are Polk, Infinity (expect to pay less than $100). Better brands are JL Audio, MB Quart etc, but these are typiclly in the $150-$200 range. Also, your Becker will not fulfill the potential of the latter - underpowered. A qualified sound shop will be able to install these for $25-$35.
I could continue for a couple of pages, yet. Almost done as much research in MB sound systems as I did in my Masters
Hope I have been remotely helpful..
farrukh