Good Afternoon All

A stiff, overhead gas-station hose snapped the fuel filler door off my X-Type a while back, and now I have the replacement painted and ready to go, but I'm not sure how to install it. There is a rigid, fiberglass-type square patch in the trunk covering the back side of the cavity that the fuel filler assy nestles into- do I have to remove it in order to remove the old broken hinge and put the new one in? If so, how do I replace the patch? It is very hard/brittle, and I suspect that I could get fumes in the trunk area unless it is perfect. Any pointers?