@Russell11 - Have you looked in the Jag Electronic Parts Catalog (JEPC), a free download off this forum? I honestly can't tell from your pics where in the boot those grommets are located.

 

I have a 2003 S Type that I bought around 12 months ago. Despite the fact that it's almost 17 years old, it's travelled only 55,000 genuine kilometres (34,375 miles). I too have water ingress into the boot and after clambering into the spare wheel well, had a friend hose the rear of the car from every conceivable direction. It appears that it might be entering the trunk space from around the tail lights. As this has only been a recent discovery I am yet to attempt the fix. However, what I wanted to add here was that I was recently watching a TV episode of 'Wheeler Dealers' where a couple of business associates buy neglected cars, carry out appropriate repairs and then flip them hopefully earning a profit in the process. An Aston Martin DB7 they were working on had wet carpets on the floor of the vehicle. It was quickly determined that the water ingress was external and not leaking into the car from the heater hoses. Ed the mechanic put a smoke machine in the car and closed the doors. Sure enough he observed smoke coming from the drivers door at the point where it meets the door pillar. He fixed the problem by inserting small pieces of vacuum hose into the pockets of the door sealing rubbers.
The point of this story is that a smoke machine placed in the trunk of your S Type may well be a more reliable method of pinpointing the ingress location of water getting into the trunk of your vehicle.

 

When I was young in the 70s I worked as a mechanic in a Ford dealership. I remember a leak in a new car that came back.. I got it the trunk with a flash light while someone hosed it for me.

 

This is a terrible affliction affecting the S-Type. I cut the bullshit, took it to a panel beater, and had the boot sealed.

 

That is exactly what the body shop did to cure a hard to find trunk leak in my S-type, some 40+ years after your experience as a mechanic...

 

I have a Rover 75. Beautiful car, and a contemporary of the S-type. I even think they were launched together at the same exhibition in the last century!!
Anyway, both cars have a lot in common, design wise. The Rover also suffers from terrible problems with water in the boot. You would think the British would have developed a handle on these issues with the dreadful weather they have, but alas, no. They take it with a stiff upper lip, a bit of water in the boot?? NEVER killed anyone!
In the Rover I took a sliver out of one of the kids swimming pool noodles. That is water rejecting closed cell foam, which means every air bubble in there is closed, and will not let water in. It thus floats, and it is also a marvelous seal, if stuffed into crevices and compressed in the cavity.
So I took a 15mm by 15mm sliver of appropriate length, and with a screw driver stuffed it into the gap between the rear light cluster, and the body, the whole way from top of the light to the bottom, facing the boot aperture on each side of the car.. With the boot lid closed, you don't see it. With it open, you can notice a 5mm strip protruding, but choose the right colour, and it is not noticeable.
No water runs off the car, into the channel next to the lid, and off the foam, down on the bumper and off the car.
I have never had any problems after that,
Takes 15 minutes, costs nothing, --- provided the kids don't catch you!!

 

To be fair, hardly any S-Types ever seem to leak despite their now advanced age.