I didnt think my car had a build sheed and Ive had it since 1989. It Wasnt under the Seat, on the Back of the seat or on top of the Gas tank. We just found it in Febuary. It was under the Driver side Bucket seat. To top it off it was made for and Sold by Modern Chevrolet in Winston Salem NC. If you look on the Parts source page it= Classic Muscle from Modern Chevrolet, Inc.
Good Luck