The LOW side lines (metal or hose) will be clearly larger than the HIGH side lines. So, the LOW port will be part of, or attached to, a LARGE diameter line. You may be able to trace LOW side to the Accumulator, which is the black coffee-can-sized cylinder in the left side of the first picture. The connector in that picture, with wire stuck into it, mounts to that switch on the Accumulator (behind the bend in the metal low side line). Jumping that should force the Compressor to run, even if the system is empty.
You have a Cycling Clutch Orifice Tube (CCOT) system. Starting at the Compressor Discharge (High Side):
Hot High Pressure R12 exits Compressor
Flows through Condenser (the "radiator" in front of the Radiator)
Exits Condenser and flows into Evaporator (the Cooling "Core" with the blower motor) This Flow is through an ORIFICE
Orifice may be in the Evaporator entrance **OR** in a crimp in the Liquid Line
All lines (metal or hose) up to this point are SMALL and any service port attached is HIGH SIDE
R12 expands (from Liquid to Gas) in the Evaporator and absorbs HEAT (producing Cold)
Cold gaseous R12 Exits Evaporator through a LARGE line which leads to ACCUMULATOR
Accumulator filters and dries the R12 (moisture may have entered the sealed system) and provides a place for any liquid R12 to settle out of the gas
Gaseous R12 exits Accumulator and is pulled into the Suction (LOW) port of the Compressor
All the lines from the Evaporator exit, through Accumulator, to Suction Port of Compressor, are LARGE. Any service port attached to the Lines or Accumulator are LOW side
HAYNES has a TechBook called Automotive Heating & Air Conditioning 10425 (1480) that's pretty informative. Also, some of the parts outlets had small handbooks from various auto a/c component vendors. MotorCraft, ACDelco, Factory Air, Four Seasons had booklets as I recall. Again, very helpful in understanding automotive a/c types, troubleshooting, repair and charging.
R12 is still available (with a License to Buy) and there are "drop in" alternatives too. Conversion to R134A is another possibility. Some conversions require that you pull the compressor and drain the oil then replace with a different oil. Some replacements claim they're compatible.
If your a/c has leaked down, suspect the SpringLock connectors. There are kits to rebuild them.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB