The codes are retrieved by connecting an L.E.D. in series with a resistor between the pin you wish to check and chassis ground. This resistor can be any value from about 470 ohms to 2K ohms -- you probably don't need the resistor, but it's good to have it.

Once connected, the L.E.D. should be ON. Now, short the pin to check to ground for 2 to 3 seconds. While shorted, the L.E.D. goes OFF. When the short is removed, the L.E.D. will turn ON again. After a couple of seconds, it will begin issuing whatever diagnostic codes are present. Each time the L.E.D. goes off is a pulse. When the pulsing finishes, you are ready to retrieve the next code by once again shorting to ground for 2 to 3 seconds.

When the system has issued the last code and another is retrieved, the system repeats the codes in the same order as before. This is how you know that you have them all.

You can erase a code by shorting the pin to ground for 6 to 8 seconds after just after it has been read.
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Talked to an engineering genius friend of mine. Says he could write a computer application that would automatically retrieve codes and could be configured to automatically erase them as well. Pretty slick. Would interface to the diagnostic connector through the PC's parallel port.