Here is a link to the same Nissan part (No. 284B8-7Y001) for sale on eBay with pictures of it outside the car and cleaned up. It appears like it should just slide off so perhaps it's just 17 years of never being removed that's the problem--try squirting some WD-40 or silicone spray around the edges. One of the pictures does look like there's a sharp protrusion that might be a retention mechanism designed to catch on to the other cover? Maybe someone with 1st gen can chime in.
284B8-7Y001 Nissan Cover-usm 284B87Y001 OEM | eBay