I've got the same problem - both driver and passenger seat heaters don't work. Took it to a specialist in Sydney to have a look.
I mentioned that from what I'd read, the heating elements can get broken from a knee in the wrong spot. They'd never seen a 159 with 2 elements broken, and thought it would be pretty unusual, and there might be another electrical problem.
They suggested swapping out the seats temporarily with ones that you know are working, and that will prove whether the problem is with the current getting to the heating elements or with the elements themselves. Mine's well out of warranty so not really worth the cost of that for me - if it is the elements, its a seat rebuild to fix anyway which is lots of $$$.