Factory fit is a westfalia detachable which drops down vertically Here's a link for you:
V Class W477 The important bit for MPs is to get the dedicated wiring kit and have it coded to the vehicle to make sure that you don't mess up reversing cameras, sensors etc.
I've used PF Jones to supply Westfalia towbars for my BMWs - the instructions are pretty good for DIY and I've done the installation myself. Not done it on the MP (have had factory fit on current and previous MP).
Hope that helps,
