Lets just make this as easy as possible.
87-88 (EXCLUDING California Mustangs, which in 1988, were MASS AIR), are Speed Density. To Convert these to Mass Air, you "have to have" an EFI Harness from a 1989 Mustang or one from a California 1988, ONLY! The only other option is to buy a Painless Wiring Harness. NOTE: The 1988 Calif. MAM (Mass Air Meter) "Plug" is OBLONG and can be converted to the Oval Shape with the proper Connector (No Splicing is required. Re-Pin it
Now, from here on in, this information is for the purpose of Converting a 4 Banger Mustang (I DO NOT CARE If it is a COUPE, HATCHBACK or a CONVERTIBLE)! AND, I could CARE LESS "What You Have Been Told"! I've done HUNDREDS of these Conversions MYSELF and "I" have been doing so for the Over24 Years!
87-89 Use a 1989 EFI Harness. How do you tell? It will have the Round Gray and Round Black Connectors near the Brake Booster BUT, NO WHITE Connectors anywhere on this Harness (except for the White 10 Pin Connector that Connects the EFI Harness to the Fuel Injector Harness).
1990 is a "one year only" Harness. How do you tell? It will have the Round Gray and Round Black Connectors near the Brake Booster just like the 87-89 but WITH 2 White Connectors on the Harness near the MAM). Build Date of the car will be within 1990 (MAYBE very late 1989).
NOTE: NO, You cannot use this harness on a 87-89!
91 through Mid 1992. How do you tell? It will have a Brown Connector near the Brake Booster. Due to Factory "Overlap", look also for the Fuel Pump Relay that will be a Black/Green "BOX" under the Drivers Seat which is the Fuel Pump Relay. If it is not there, you have/need a Mid 92 through 93 Harness.
Mid 92 through all of 1993. How do you tell? Look for the Brown Connector near the Brake Booster but also Confirm that it Does NOT have the Black/Green "BOX" under the Drivers Seat. Also, Confirm that there ARE 2 Black "Boxes/Relays" by/near the MAM.
QUESTIONS? Feel free to e-mail me at firstname.lastname@example.org
Don't bother trying to make it sound like you want to buy something, just tell me what you want to do, Buy, Sell or Confirm Information.
YES, I have ALL of the Harnesses that you will need and/or Information!
"Information is always FREE but Stupid Mistakes are Often Costly"
Thanks All (and the Corral too)