Looking for parts in an unsupported environment - I feel for you, and your father-in-law...I have owned a handful of different Renaults in my past, so BTDT.
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(UK) Clementine's Renault 4 Garage
http://www.renault4.co.uk/
http://www.renault4.co.uk/forum/index.php?
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RENAULT 4 - Links
http://www.milons.co.il/renault4/links.htm
http://www.renaultownersclub.org/links.html
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OK, high praise on Clementine's Renault 4 Garage forum for UK's Whitehouse Renault, George Thompson.
http://www.renault4.co.uk/forum/showthread.php?t=47
"George Thomson is exceptionally knowledgeable and knows Renault 4's inside out. I've recently completed restoring my 1984 R4 to virtually as new and Whitehouse supplied all the parts, gave excellent discounts and back-ordered obscure bits and pieces from France."
http://www.whitehouserenault.co.uk/
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'Hope Stijn's lead will produce for you.
Good luck...what a PITA, eh?...whew!