Check for voltage at the element wires. I imagine rear buttons are like the front and a light comes on when you press the button? If there is no light then maybe start checking in that direction. looks like power for rear seat heaters comes from unloader relay so maybe check that.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)