That empty plug is the diagnostic port. If you have a check engine light on you should be able to check it with a volt meter that has a needle or a 12 volt LED, pin 1 on the port is positive and pin 12 is negative.
Or if you have a palm pilot or want to buy one (ebay has some still), then you can buy the cable for it and install the software and read codes and other data electronically. Its been 20+ years since I had mine powered up so I cant remember exactly what the limits on it was but it was pretty basic compared the new computers these days.
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