OK so there are some differences as you can see by the pictures of the MB FOB vs the original.

There are differences in both the plastic case and the metal key part that need modifying.

The first difference and obstical is the original plastic back has a single tab for the white piece to fit over and the newone has a T shaped tab. I cut the tab with a razor knife to allow the white piece to fit properly (see pictures).

2nd issue in next post