We have heated seats and the on/off button is on the right of the drivers seat (LHD) and the left of the passenger seat. We may be going to the van in storage and if so I will take a photo. Alternatively if yours is available post a photo and I can comment.
Thank you for the suggestions. I've tried to put a picture to show you but a message is telling me "the uploaded image is too big".
The button is only on the left side under (not on) the seat so I think Markman is correct.
Yes I quite agree. On mine I think the button is dark grey with a red light showing when on. I too think the waste tank heating element only comes on at a low temperature. We have never used it as we aim to get rid of waste regularly and I had a second outlet added to our waste tank at the near edge where there was a suitable exit point.