manual fuel door opener in trunk? Exactly where in trunk please please?

Open the trunk, pull back the carpet (or plastic cladding) on the side with the fuel door, and pull on the cable (or cable lever) that releases the fuel door.

1968 was also the last year for the "stacked" dual headlights, which were replaced with side-by-side dual headlights in 1969. In these early days of Cadillac and car production there is no emergency release lever from which the trunk can be opened.