On my late '86 Ser.1 Sierra XR4x4, the splashguards / arch-liners at the rear are so brittle they
break if you even look at them !

I'm assuming this is a common problem, so I'm reluctant to spend much time looking for other 2nd-hand
ones that might suffer the same fate.

What's the usual fix for this ?

I would be happy to adapt a liner from another car, or if I could get some decent original Sierra ones, I would
make some in fibreglass, and use these....

Any suggestions ?

Just to be sure.....it's a 4-door shell.