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It'll be an EEC-IV code reader. You can find one on eBay, I just got one. It will take some time to find one for a decent price, as most are rediculously expensive. I paid $18 for mine including shipping from AK to TX.
The plug may have a cover on it labled EEC TEST, mine was attached to the fuse box under the hood near the battery. Yours may be located in a different location.
A paper clip is much cheaper than a code reader if you don't mind counting blinks on the check engine light. Just search google for "eec-iv self test" and follow the instructions...
Since I have three vehicles, all different years and OBD styles, I broke down and bought a OBDI/OBDII scanner. Gone are the days of finding the blasted LEDs and/or watching the CEL afraid to blink.
They are awfully handy if you are going to work on your cars regularly. Also handy in obtaining free cases of beer for scans!