Look on your drivers door sticker, under Int. Trim. You can refference it to this color code list for 1991 Mustangs.
INTERIOR COLORS/INTERIOR TRIM CODE
- Titanium
Cloth = FA, Vinyl = BA
Cloth&Vinyl = DA, Leather = CA
- Crystal Blue
Cloth = FB, Vinyl = BB
- Scarlet Red
Vinyl = BD, Cloth&Vinyl = DD
- Black
Cloth = FJ, Vinyl = BJ, Cloth&Vinyl = DJ
- White/Titanium
Vinyl = BL, Leather = CL
- White/Red
Vinyl = BN, Leather = CN