The NTC thermistor increases resistance as temperature falls. If it is open circuit, the PBU thinks the evaporator is at an extremely low temp.

My sensor was about 14.00 ohms. I got another one - new style W201 that is has the metal casing and ohms at just under 7. I tested them both at the same time, same outside temperature.
Still no compressor kicking on.

You have a PBU problem from the sound of it. You already know that grounding the pin at the PBU turns on the clutch, you know the Evap sensor is good, the PBU is all that's really left at this point.

I got the PBU already sent out via UPS. Stay tuned for the installment!

This has become a nightmare.....
Okay so the first set of replacement PBU arrived in a box that was driven over, the tire print looks like a forklift tread, not joking. Contents smashed. ( Never use the USPS for anything of value! )
Next replacement send via UPS. Box looks good. Strike 1 - all the airflow buttons are depressed?! Strike 2 - there is a rattle inside if you shake it. Not like its something completely loose and rolling to one side to another but as if its contained in a small area, like a a baby toy rattle.
What a heck lets try it. I push around the buttons some and they spring back out.
Plug it in.........................Strike 3 - Still. No. Compressor.
Helppppppppppppppppppppppppppppppppppppppppppppppppp!

Is there any other way to test the PBU and or the ground as to where it grounds the A/C compressor clutch?

You already tested all the wiring up to the PBU and it'll engage the compressor when grounded. That rules out everything up to the PBU. There's something in the PBU that's preventing engagement, that's as far as I'm useful in helping on this matter since I'm not familiar with the operating conditions of the W201 PBU.

How about where does the PBU get its ground from?

Okay so the better part of this month was wasted with trying to get PBU delivered to me - in one piece. From the first vendor I got 2 damaged ones and a third that was lost in space never sent. Now I got a quality set from George at Performance Analysis.
However- I still have no compressor engagement in any setting, defrost too.
Now all the other functions work: dash air going where it should, fan speeds.
**** How about where does the PBU get its ground from?

Just like to say thank you to you and everyone else that gave some input on this Major headache.
Also Performance Analysis | Climate Control | Diaphragms | Blower Motors | Pushbutton Panels | Vacuum Door Locks | Autotemp | Cruise Control | Engine Oil Analysis | Mercedes-Benz Parts | Mercedes-Benz Club of America | Oak Ridge | Car Care is a great company.
I had to find an independent shop that would test out this car. https://www.facebook.com/autowurksdiesel/
It even took them a long time but they found it and had to use the test cables brake out box for the W201.
**** PROBLEM SOLVED!
Turned out to be a poor solder joint connection in the right side / passenger side harness of the push buttons unit that came from the a temperature sensor. When the push buttons were exchanged sometimes - just sometimes - the A/C would work for a bit and then gone. It did not look obvious and was only found using a meter.
I'm ecstatic this car is now 4 seasons capable!

The craptastic German soldering skills claim another victim!