I've seen a 2009 Forester 2.0l, manual petrol that I like, for sale.  Vendor says the heated seats are not working though. As someone who benefits from the heat on my lower back in the winter I'm keen to have this feature. My question is; is heated seat failure common? If so, is it usually an easy fix or is it likely to be a fault within the seat itself? ( not so good).

if both seats not working and on both heat levels then it probably not heating elements or the seat sensor faulty unless one bad seat blowing circuit protection .

If seat element faulty or the seat sensor it generally easier/cheaper pick up a used working seat .

things to look at are if seat heat switch illuminates as if not the first issue is control side or power. if switches show life then it either seat or wiring  (wiring can get damaged if people sticking stuff under seats or bt rear passengers feet)

 

Kind of following on from this with my own related problem, my passenger seat is ideal and turns to lava when required, the drivers seat is very subtle by comparison...anyone have any thoughts?

each seat has pair of elements, your drivers set probably only 1 element working on high heat .

So back to your suggestion before, probably best to find a replacement seat if I want to fix it? I mean it doesn't actually bother me all that much, I'm quite warm blooded anyway and prefer the car cool... but just good to know..

once checked switch wiring and power earths values (can also ohm test the heater elements) then good way go is used seat, even if strip down you can't repair element only wiring to it and seat cushion switch.
As a shop I can't strip them as becomes too costly be viable and no way know if even repairable so replacing seat way to go in cost and turn around .