My '98 XJR does not have cavities rustproofing. I have just reconstructed the RH rear floor area (where the jacking fixture sits) with new internal structures, captive nuts etc. The rest is good but I do not want to leave it all unprotected. Where can I find the schema showing where to drill holes for injecting antirust protecting fluid?

 

I would recommend inner/outer sill protection. There are cavities that run along the car front to rear.

Put the car on stands. Remove all 4 wheels. Look at the little triangular panel that covers the sill ends at the base rear of the front arches, and base front on the rear arches.

use a small drill and form a hole near the top of the end plates to feed the treatment line all the way in, then depress the plunger and slowly draw the hose back out applying surface treatment as you go.
Once one side is sprayed and treated seal the cavity with a small grommets front & rear - this way you can apply a coating each year as standard maintenance.

I recommend this product, it's far superior to Waxoil.

https://bilthamber.com/product/dynax-s50/

Easy to install, developed for oil rigs and a bit special at keeping rust away.

 

Looks to be a pretty good product. I guess a sealant might help against further corrosion.

 

That looks a lot like an offshore product from the 80'ies WireLube. I have ordered one can and will compare to what I have used the last 10 years: Lanolin based Lubrifilm. The great thing about the latter is that it is clear so you see what is happening beneath. A bit messy if you need to work in an area treated with it but easy to clean off with a mild solvent.