Bingo, found it!

Looking at the A/C system as baseball diamond really makes it very simple.

I was having a hard time since the layout is a bit hard to track due to the lack of space under the hood. It turns out the high pressure port is located on the engine, underneath the plastic cladding.

Next, is to pick up a manifold gauge set.

Thanks!

__________________
1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500