AC low pressure port location

USA are you or Europe???? hint cars not the same.world wide.
Mercedes-Benz ( the makers here have no service manuals sold to owners) <<< makes repairs hard and expensive.
R12 OR IS CAR UPGRADED TO R134A OR R1234?

well it has 2 caps on the port. so 2 there are
there are 2 lines there, one is huge one is small
one side is called low side, and the the other is high side.
best is not touch the 300PSI (21 BAR.) SMALL LINE TUBE.
do not attach a walmart R12 can here or it will explode. in the face, OK end safety rant1, and where goggles.
the low side is about 10X LESS ABOUT 30 PSI
THE R12 RETURNS HOT AND IN GAS FORM. USING LARGE TUBE.
THE COMPRESSOR COMPRESSES THE GAS BACK TO A LIQUID STATED, AND SEND THIS LIQUID TO THE CAB EXPANSION VALVE TO GET COLD.

ONLY MESS WITH THE LARGE LINE , PORT
R12, IS FACTORY PORT,

THIS IS ORIGINAL 1999, WHAT IS THERE NOW I'VE NEVER A CLUE BLINDED IN A TEXT BOX. HERE.. MESS ONLY WITH LOW SIDE. OR BOOM,
THE HIGH SIDE CAN EVEN HIT AND EYE AND BLIND YOU.
EASY. GOOGLE ARE NECESSARY.

IN REAL SHOP THESE 2 ARE CONNECTED TO A TOOL CALLED A MANIFOLD. AND IS USED IN A SAFE MANOR.
A SKILL TECH TRIES NOT TO INTRODUCE AIR TO THE LOOP
AND DOES SO PURGING LINES IN THE MANIFOLD FIRST.
AIR IS BAD, BAD NEWS. (AND AIR IS EVERY WHERE)<LOL

check connector --wiring harness --ground--for short or fray but most likely connector will be problem

A/C is cold or hot?

You need to have the refrigerant charge level checked, it sounds like it is low. You Need a set of AC pressure gauges to do this testing. If the system has a low charge the pressures will be very low in the low side testing port.

open engine hood its at the right front on the ac condenser it's covered by black plastic flap open the little flap you can see the low pressure valve in it.

start at ac pump and follow the lines/hoses off the compressor the large line is the low side with a small service port the high side srvice port is on the small line/hose it is the large port(and they could be on the compressor

One should be by the drivers side strut mount and the other up by the Aux fan in front under grille.

look for the large ac line/hose and follow it untill u come to the small service port this is wher u charge the small ac line/hose has the large port this is the high side

AC charge, AC dryer, Low pressure sensor, High pressure cutout, engine temp sensor, AC compressor bad, AC relay bad. CCU can be bad but I doubt it.

If there is no gas in the system it as you pointed out the low pressure switch will shut the power to the clutch in the compressor off. your pressureswitch should be located near the dryer at the front of your Benz. You could try and bridge it to see if the compressor engages, how ever DO NOT try and keep it engaged without gas in the system or you may find yourself having to change the compressor again.