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You should have see my original black carpet in that area - faded to gray/purple. I live in the sunbelt, but it really didn't see that much time parked out in the sun. I think that area catches more sun than the floor, but who knows, maybe your front has been replaced.
The materials used by GM back then, did tend to change color and/or fade. The carpet on the bulkhead, looks to have faded, and I think the quarter trim panels, window frame and retractor covers may have also.
At first I thought that the door panels had been replaced, but normally you don't see those arm rest repair covers, on new panels.
My vote is that it has been replaced...or has had interior paint applied on it.. If it has faded...the fading it way too even...NO light and dark spots. The color is way too even for the parts behind the seat.
AND without you taking parts out...that would greatly tell the story. It is EASY to tell if the parts are factory or aftermarket.
Seats, door panels, etc. are affected more by sun than the stuff in back. And, they are different plastic types. I think the trim in rear of car looks more like stock color. You can re-dye interior parts easily for low $$$.
But, even when new, the various interior pieces were not identical in color. Several vendors for interior parts...each slightly different.