If your ML is a post facelifted one then it really is plug and play. If you have a CD changer then you will need a small loom adapter nad possibly an antenna adapter.
The factory antenna can go on the roof, the cabling is in place and re-appears behind the existing audio10. You will need to cut two black lugs off the back of the double din housing. Connect it all up and switch it on.
20 minutes tops!