Auto Air Conditioning & Heater Repair: DIY a/c recharge, auto parts store, washer fluid

Question
I have 2000 expedition w/5.4liter engine. Over the past 12 months the a/c has gradually become less cold suggesting it needs to be recharged. I want to charge it myself but the recharge hose coupling that I bought at the local auto parts store does not fit what I beleive is the low side port. This is the port that has a black cap and is next to the washer fluid container. Is this the right port?

Also, is there any reason why the front A/C would get colder when the rear a/c is turned on?  

Answer
HELLO REED, A SIMPLE WAY TO LOCATE THE SERVICE PORT. GO TO THE COMPRESSOR - AT THE REAR ARE TWO TUBINGS- THE LARGER OF THE TWO IS THE LOW SIDE-FOLLOW THE LINE  TO THE ACCUMULATOR. SOMEWHERE ON THE LINE THE SERVICE PORT AND IT COULD ALSO BE ON THE SIDE OF THE ACCUMULATOR CAN BODY.BE CAREFUL WHEN ADDING FREON AND WEAR EYE PROTECTION.A DOUBLE SYSTEM TAKES MORE THAN A SINGLE UNIT.IF YOU NEED TO PUT IN MORE TAHN TWO CANS OF FREON THERE`S A LEAK GOING ON..