You are correct that it should be under the dash. I am attaching info that you have probably seen.
https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method
If you are not finding it, then I suspect someone moved it to maybe under the hood. I have seen some people move them because they don't like working under the dash. So of the older people or larger people don't like too much climbing under the dash so they relocate them. I would look to see if there is a group of wiring that looks like it is headed out towards the engine bay and follow it.
If it is not there, can you get some pictures of the under dash and we will see if anything looks out of place or like it could be a clue?
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Monday, November 16th, 2020 AT 3:19 PM