starter just got replaced a week ago & now the car won't turn over? What to do?

If the car won't crank at all its doubtful its any sensor for the engine. It could be part of the smart key ignition system. There are also many modules on this car that need to communicate with each other. If that communication is broken it will cause a no crank issue. You will need someone with the STAR computer program to run a scan on the complete system. This can be done at the MB dealer or a good Euro Independent shop. Here is a link to find a shop close to you. https://repairpal.com/repair-shops Alex