Up to the 1986 model year, mustangs had a low fuel light warning module, but it was in the dash harness. It could possibly be out of an 86GT. I know the harnesses changed a couple of times over the years. I recently did an EFI swap into my 84 GT and found through wiring diagram research there were differences in wire colors and connector locations.
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Factory Five Roadster Build
Yellow T-roofed 84GT
(In users rides)&
20th Anniversary GT350 ragtop