The "Service Engine Soon" light flashes when reading the codes not the yellow amber brake light. I believe on the 1991 Reatta [I don't have one] you still have a gray "ALDL" box under the dash that you have to jumper the pins. Directions how are in the "How To" that Ronnie has on the top of the page.
This is one of the best topics we've had on ROJ in a long time. It was in the "What did you do with your Reatta today" topic but I moved it because it really deserved to be a topic all it's own. Thanks 2seater for allowing us to follow you on your journey of bringing Dave's old Reatta back to life. Good job!