Hi
Common fault on the s-type, all the bulbs have a permanent 12v feed one side of the bulb, other side goes to rear body module which switch the earth to bring it on
so if you touch the body while removing the cluster or accidentally putting the bulb in the wrong way round will pop a fuse
also make sure the bulb is correct 21w4w, but sometime they give you a 21w5w, which will go in but pop a fuse
theres 2 fuses to each rear light, make sure the fuses are good
there are no earth at the rears lights only positive feeds, as above, one side of bulb has a permanent 12v feed, earth is switch by the rear body module
cheers
Joe
if you need wiring diagrams have a look here http://www.jagrepair.com/