When I have a hard time finding specific information online about something, I like to throw up a note on a forum for someone
to find later (or myself years from now, it's happened). I'm long winded, fair warning.

90 XJS, manual seats. The air lumbar was dead and per usual the hoses were crumbling, the odd little leather baggy they house them in was full of what
used to be foam and turned to dust, the hose between the overpressure cut-off valve and the pump was toast, and in my case the air bladders themselves
had holes in them in un-patchable places. Remove, clean, replace tubing, replace bag with a $20 commercial air bag such as this:
www.amazon.com/dp/B088F316V6
The larger of the three is a pretty reasonable fit and all but indestructible
You can actually slide this in, due to it's having a thin piece of plastic inside it such that one can slide it into
tight places, without doing much more than pulling the seat back off. I put a piece of doubled over outside
heavy duty duct tape on it just to keep it in place but I doubt it would have moved.
Problem seems to be solved.
I don't like air lumbar; a mechanical mechanism is the way to do it as it's always where you last had it adjusted to
and will never slowly leak down. I'm not positive my driver's side isn't doing just that, have to pull the seat back off
again and observe it as it's pretty hard to tell subtle changes, least to my back.
Passenger side it occurred to me to bench test it at full pressure, all good.
I'm still not sure how they managed pump up and release out of two wires when release is not reversing the pump/polarity.
But it works and it was cheap and the bag will never fail again, so yay.

Seat heaters.
If you haven't guessed, I have a bad back, so lumbar and heat are a thing.
My most driven other car is a Lincoln Town car if that says anything.
And I'm not old.
Anyway, the seat heater program on these sounds like an annoyance, my understanding is it won't come up if it's
over 60F or so, is time limited and you may not be able to re-enable them unless the temp is again below 60F or
whatever it is. I'm sure it works, but not how I need it to. (again, Lincoln town car, excellent heaters)
So, fixing. In typical fashion the bottom heater pad was shot on both, the back was good.
I replaced them both since I wasn't confident that the factory back pad would tolerate being on for
hours, or if it had a thermostat of its own.
Usually, I buy factory pads since they fit exactly but that didn't seem to be an option here, and
wasn't quite what I wanted anyway. Dorman makes a pretty good upper and lower set with wiring
and switch (high/low) for $50ish a seat that I've used a few times with good results.
Seat Heater Assembly | 628-040 | Universal Seat Heater Element | Dorman Products
I actually got these for 34 bucks a set on ebay new, sometimes you get lucky.
There is probably a better suited set out there, but I had no idea the size or what the factory ones looked
like going in, so these I got. I can tell you the lower pad is just the center section of the seat, the outer bolsters are completely separate from
the center, and it does not extend into the thigh wedge area. The seat back is similar in that it's the size of the center piece, not the
bolsters, and goes up pretty high on the seat back, for reference. I had no idea the back and bottom weren't complete solid pieces
of foam, learn something new every day.
Anyway..
The real joy of this job, and kudos to Jag's weirdo seat design team, is you don't have to pull the skins
off to do this, which was my original question I couldn't find a good answer to.
On the back, once you pull the panel off and remove the vertical strapping (be gentle, it's ancient) and maybe the horizontal
panel that holds the lumbar support, you can weasel your hands into the center foam block and pull it back enough
to slide a new pad in. I never remove factory pads, they are glued down like crazy and tear up the foam too much, just
stick the new pad on top of the old one and call it good, you'll never notice unless you have some serious princess and the pea
thing going on. Make some effort to affix the new
pad so it does not slide around, but it's unlikely if you install it toward the bottom as I did.
The only sub-optimal note is the factory back pad is huge, extending way higher than these square aftermarket ones.
I don't think in practice it will be a big deal (least for me) as I really want heat down low where my wonky vertebrae are, but it's of note.
If I'd known ahead of time (now you do) I might have hunted up a taller pad.
You can also google up the Dorman pad size and note that they are juuuuust about too wide, top and bottom, to fit
in the space allotted. You generally don't want to bend seat heater pads much if at all, but it happens, and these fit
well enough after some monkeying around and splitting of differences. Always check resistance on pads before and
after install to prevent later heartache.
So, the back is a breath of fresh air compared to pulling a dozen hog rings and trying to remove 30 year old leather, just
gentle the foam back and slide the new one in there, affix lightly, run the wire in the stock loom.
As mentioned, both my rears were good, I secured the factory wire to them in the seat back with zip ties for future
possible use by someone. I try my best to never butcher things, who knows what some poor sob is going to be trying to
do a reference restoration on decades from now.

The seat bottom is more annoying but doable with the same basic procedure.
Pul the big rubber pad, odds are your driver side could use replacing, mine was surprisingly good for 240K miles
but it's had non-giants sitting on it most of that time so your mileage may vary.
Once the rubber is out (so weird) you can again weasel around from the front and find your way into the cavity between
the leather and the foam and you'll see the dead factory pad in there. You can also cut that wiring off flush as you can
just to get it out of the way.
Side note, my driver side had a really neat occupancy sensor consisting of two pieces of copper separated by some foam
with a couple of wires to them, which were broken since they soldered thick wire to a piece of copper. But it was a neat idea,
and the passenger side did not have such. I don't know what's up with that.
Anyway, you can get in from the front above that foam and mostly, carefully, if you have small hands, get the seat heater slid
in there. Failing that you can pull some hog rings and break some glued foam on either side to have better access.
Remembering where your heater grid cord needs to exit, and again making some effort to affix it from moving around.
Not much choice in positioning, it barely fits in the space allotted, especially from side to side.

Sooo, there ya go. I have a few pics of this stuff, but they aren't a step by step by any means, it's really pretty self-explanatory once
you get in there, but I'd have liked to know what was up beforehand.
I've got a heater core to replace in this thing sometime between now and winter and I'll pull the center console then and
run the wiring for these, I plan to put the switch in the factory position. I'd have loved to keep the factory switch, but it can't
readily do the high/low setting. I'll make an exception in this case.
Also, what likely took longer than the lumbar and pads was pulling the carpet and insulation and shop-vac'ing all the
crud up and treating a couple of minor surface rust spots on the floor. Be prepared for such unless you don't care or
have a super minty never seen rain or snow-covered feet kinda car.
And omg those front seat bolt nuts.....
I don't even know what to say about them, I think they need some backing to rest on to keep them
at about the right height and orientation. They don't have that and on the inner ones you can barely
get to them for the carpet. You'll want a little magnetic retrieval tool, and I ended up clamping them
with a long nose locking plier after a loooot of monkeying around.
If there is some silly trick to them, I'm all ears.


Edit:
Now is also the time to modify how this seat sits if you don't like it.
The bottom is held up by that rubber pad, which is.. Creative.
Looked to me like one could put straps of some sort Left/Right hung
off the metal rods that the hog rings are in and adjust tension with them
or add some more padding of what have you.
If I had a raggy rubber pad and no way to replace it I'd consider that just
do some test-sitting before buttoning it all up. You could also likely clean that
rubber pad really well and use some old tire innertubes and correct adhesive
to re-enforce or repair it depending on where it's gone south at.

On the seat back, those vertical straps, again mighty creative on Jag's part.
When you sit back the seat back moves a fair bit and sort of rests on those straps.
One could replace them completely or add (memory) foam between them and the
factory foam wherever you feel like it's lacking, if anywhere. Again, test fit, but you can
do the back with the seat in the car at least.

Oh and man these seats are light, I was really surprised.