To be honest Brian, I've repaired a few of these now, and opened up a few more whilst saving the panels, and I'm still not 100% sure why the rear sections corrode so badly ?
Any box section on a vehicle should always have a drain(s) of some description at the lowest point - either a simple drilled hole with the burr removed (as 75's generally have along the length of the factory sills), or a proper pressed outlet to let any moisture escape. Equally, a decent wax injection spray covering the internal surfaces should be applied to prevent rust forming.
The jury's still out, but I tend to think that a missing plastic jacking pad allows mud & muck to enter the rear sill chamber at the base of the jacking bracket, collects inside and subsequently holds moisture against the outer sill skin. The factory wax injection is now getting old and if dried out, the rust will start.
However, an equal argument would therefore be 'Why doesn't it happen at the front then ?' In answer, I don't know.
However, what I can say is that the sill section ahead of the foam acoustic diaphragm always seems to be completely rust free (assuming it's not been damaged & badly repaired.....but that's a whole other story....
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A few years ago, whilst repairing the sills on my Range Rover Classic, I made a small tool which when clamped in a vice, presses a 'factory' type drain into a flange. Although not strictly original, I've used this on the new sill sections on S542AOX, directly below the bottom front corner of the rear inner arch to allow any water that gets in around the wheelarch flange to drain straight out, saving it from having to sit around the jacking point. Ask me again in 20 years whether it's worked....
