Hi Bill
Yes - the model pictured there (1/16 Revell SS100) has leather seats. You can get leather from all sorts of sources (soft bags, wallets, gloves etc from charity shops are a great source). Choose the softest you can find and cut a piece oversize for the application. Turn the leather over and thin the back with a modelling knife till it is not much more than paper thickness (be careful you don't cut through). To stick the leather I use an ordinary household glue - smear the part with glue and apply the leather, stretch it and fold around the edges and trim as necessary. Use the rounded edge of a steel rule to push the leather into the cushion folds if it is that type of seat. Here's another pic of that Jag with the roof off so you can see the seat cushions.
And going back to your original question - the tonneau on that model was done in the same way as I described before except that this time the original plastic piece was covered with clingfilm before covering with the material. When the glue/paint mix was dried the whole piece was then removed from the plastic and in this way it gives it a softer form (you can do this with roofs too but you really need a frame to fit it over for best effect).
For more information on the leather there is an excellent tutorial on Alex Kustov's site at http://italianhorses.net/Tutorials/Leather%20101/lthr.htm
Hope that helped a little.
Martin
http://www.freewebs.com/martsmodels/