Where is the low ac suction side located on a 1992 buick regal

First find the A/C compressor, it is attached directly to the engine and has a pulley on it. When you turn on/off the A/C while the engine is running you should be able to locate it by listening for it to engage.

When you find it you need to trace the two metal pipes that come out of it. One pipe is the high pressure side and one is the low. You will know which one is the low side when you find the 'tee' with a plastic cap (both high and low have one).

The fitting that comes with the A/C recharge kit will only fit properly on the low pressure port. Sometimes, but not always the low pressure port has a blue cap.