Where is the diagnostic plug ?

it is under the hood on the passenger side. it is not and obd 2 system. it is obd 1 and requires a scanner to pull codes.

Here is a guide to help you.

https://www.2carpros.com/articles/ford-lincoln-mercury-obd1-1995-and-earlier-diagnostic-trouble-code-definition-and-retrieval

Please run down this guide and report back.

Roy