Just one faulty element of the six in the seat will render the heating inoperative. If you know your way around electrics and a wiring diagram you can check each element separately and then bypass the faulty one. The connections for the elements are under the seat and in the seat back (remove the back panel, two screws underneath and pull the bottom out and lift up).
After buying a cheap replacement new cover in the incorrect colour just for the heating elements, I unpicked the heating pads ready to stitch into my covers but on investigation I only had one faulty base side element so just bypassed it. Toasty bum since then!

Learning a lot on this thread... swap the seat covers around and six heat pads, I presumed there were just two...

Cheers Craig and Weejock...  Pete

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2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
-Click for Project Fatty off roader-

Have a look here: http://www.fullfatrr.com/forum/topic22581.html , I posted some pics of the seat heaters unpicked from one of the covers. There are six separate heater pads with individual connectors for each but as they are sewn into the seat covers they could be classed as two.
The main ones that fail are the obvious ones that get the most abuse, the base side bolster and main seat base but that is not always the case. They are wired in a series circuit so if one goes open circuit they all go down.
I seem to remember the main seat base element has the temperature sensor, that's the only element you can't bypass as you would have no temperature feedback.

To be honest if you took it apart and saw the damage from a failed element you would only use the heaters very sparingly, they can burn through the seat.

Have seen several early cars with a big brown burnt patch in the leather!!!

Dam, I wont be buying a curry from your neck of the woods  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!