An ascMcLaren forum for 84-86 Capri and 87-90 Mustang based cars modified by Automotive Speciality Corporation, a Division of American Sunroof Corporation. (no affiliation)
I really have no idea on the 1990 models, but the 1985 merely requires removing the buckets behind the drivers seat and passenger seat to get access to the retraction mechanisms for the seat belts. Good luck!