noxioux fumes in car / need help

Does it smell like burnt exhaust fumes or raw gasoline?

There is a vacuum at the rear of the car while you are driving, so make sure the trunk lid seals are in good shape and that the lid closes/seats fully; also check tail light gaskets, antenna gasket, grommets, look for tears/holes in the trunk sheet metal that weren't properly sealed after collision damage, etc, etc... any air leakage in the trunk area allows exhaust fumes to enter. Also make sure your exhaust pipes/extensions extend rearward as far as they did when the car was new.

If the smell is of raw gasoline, you are looking for fuel leaks... I'm not fully familiar with your year/model, but some Jaguars have the fuel filter inside the trunk so if the fuel hose connections leak you'll smell fumes. Another possibility is the filler neck hose connection, as are possible tank or fuel line leaks.

You are not in the habit of stopping at roadside fruit vendors and picking up a load of durians, are you?