i just ordered the factory electrical troubleshooting manual for my 93'. its not here yet, but i know it shows the locaton of the EEC relay. I dont know if its the same in your 86, but its worth a shot. E-mail me, and when I get the book(should be soon)ill let u know. Other than that, I belive its under the dashboard somewhere.
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1993 GT/AOD
'93 Mustang GT