High side  goes to the condensor (which is up front behind your
radiator. Low side comes from the evaporator (that which gives you the
cooling air.).

On my 91 Westy the low pressure fitting is right by the compressor,
while the high pressure is up in the cabinets.

On my 86, the low pressure is, again, right by the compressor, while
the high pressure is off to the passenger side.