Koleos Rear Window Heater Failure

It's possible a weak battery could be the culprit. Modern cars are very sensitive to voltage drops. The rear window defroster pulls a lot of current. Have you had the battery tested recently? What's the voltage reading when the engine is off?

Voltage reading? I'd have to check that but if the defroster pulls that hard, and the windshield one is also affected, does it mean that the battery is the only possible explanation? Is there any other way for the auto defrost to fail?

While other issues could cause similar symptoms, like a faulty relay or wiring harness, a heavily discharged or defective battery is a very common cause on the Koleos, especially with those kilometers. The best course of action would still be to get a professional diagnosis at a workshop. They can perform a load test on the battery and check the entire circuit.

Thanks a bunch for the insight! Took it to the shop like you said. Turns out the battery was indeed the problem, putting out next to nothing. New battery and all's good. Cost me 290€, but at least I can see out the back again!